First Bread didnt rise......pic attached

Can you tell us about your starter? How old is it? What is the hydration (I assume 100% based on the formula being used)? How is it kept?

I think 1200 grams would be too big for the pan. I seem to recall that pan size using a standard 800 grams or so. Barring any issue with the starter (or over fermentation of the dough), I would say try it with that amount. So you could do a single loaf using 125g of starter, or two loaves using 250g starter and the same 1-2-3 proportion. This will give 750g of dough per loaf not including the salt (which should be 8 or 9g per loaf in this case).

Speaking of salt, that can make a difference as it helps to slow things down. Do you know how much you used? It should be about 2% of the flour. I know a lot of people do the 2% based on the added flour and have good results. Personally I always base the 2% on total flour including the flour in the starter. The difference is only about 1 g per loaf normally so it really won't matter for how it works in the dough (just the flavor) since it is really the difference between roughly 1.8% and 2% which is still in the typical range.

In the case of 125g of starter this would be 8g or 9g of salt (I would use 9g personally but that is the flavor I prefer). Obviously double that for the double loaf size. You could go as low as 7g without real structural change if you like less salt. I don't think I would go below that though. 10g might be pushing it flavor wise but would still be okay as well.

ETA: This is a fairly high hydration dough so you might just need to use more bench flour until you are used to working with that wet of a dough. It will change it slightly but you shouldn't notice much difference other than in the crumb structure.

BTW, you can make a full 800g loaf by using 133g of starter and 10g of salt. I was just trying to make the math/rounding easier, but it isn't like 133, 266, 399 (plus 10g of salt) is really that hard to figure out (or however you want to round it). One thing I like about this is while it is easy to say I have x amount of starter, it is just as easy to say I want y amount of dough and divide by 6 to get the amount of starter.

for the first ever sourdough loaf from a new starter.  

Now I think you should repeat but reduce the amount of ingredients for one 800g loaf.  Since you've been thru this all already it will come so much easier!  

Lack of salt will reduce the volume of the loaf.  

Yes, reduce the water or hold some back while first moistening the flour.  Why? It is easier to add water than flour because all the ingredients are based on the flour amount.  If more flour is added then one needs more salt, more butter, more etc.  You get the idea.  

Different kinds of flour absorb at different rates and some more than others.  Pure AP for example will absorb less than if there is some whole flour mixed in.  Rye will also absorb more than whole wheat.  Whole rye even more than fine rye.

Proofing and sourdough.  A good way to learn about sourdough and rising is to work on a table and cover the dough with a glass bowl between folding.  Knead or just moisten the flour and let it sit for 20 to 30 minutes, cover.   Uncover and lightly flour the tips of your fingers to lightly knead or fold the dough.  Shape and cover.  When you see the dough rising more out than up, remove the bowl and fold again to retain shape.  Cover.   Sourdough will tend to ferment and loose it's shape faster than a yeasted dough so that is a major difference.  The bulk ferment is more like a big long rise with lots of interruptions.  

It often helps to pinch off a large marble or pingpong ball size of dough and cram it into the bottom of a small narrow glass, like a straight sided shot glass.  Use this as a rise guide.  Mark the level and when it "doubles," cover to prevent drying.  Keep it next to your working dough so they share the same temps and you can refer to it easily.  When the dough starts puffing up, do a fold but not on the little ball.  That way you can deflate the dough yet know how far the dough is along the "rise."  Don't let the sourdough gauge double before you go into a final shaping.  You can also knock down the little sample and let if final rise.   Also, don't let it proof completely to "double" and save some rise for the hot oven.  

You will notice that as time and fermentation go by, the sourdough dough will seem more fluid and softer.  It will also take less time to loose it's shape.  When you find yourself folding every 40 minutes to keep it's shape, put it into a well floured banneton for the final rise and deflate the gauge.  Turn on the oven and watch the rising dough.

fit the pan just put the pan on the scale and fill it with water.and see how much the water weighs.  If you want the dough to eventually double when baked and be above the rim when done so it looks like a proud loaf then take the weight times .6 (point 60 or 60%.  That is the weight of dough you need. This will fill the loaf pan 60% full.  To find out how much starter by weight you need.... just divide the 60% number by 6. (1+2+3=6)

When it rises to 90% proof (when you want to get it in the heat) the middle of the loaf will have risen above the rim level by an inch.

Then put it into a non convection 450 F oven (your heat is too low) with a metal pan of water in the bottom, half full of water with a kitchen towel rolled up inside.  Put the pan in the bottom of the oven  when the oven hits temperature and wait 15 minutes before putting the bread in to bake.  Do not put your face near the door as you load the bread - let the steam escape first or you will burn your face off. 

Make sure to PAM spray your loaf pan the bread is in.  Steam for 12 -15 minutes then remove the steam pan. turn the oven down to 425 F with convection fan if you have it and continue to bake for 15 minutes.  With an instant read thermometer test the middle of the loaf and shoot for 205 to 210 F.  If you want the crust on the side to be more brown then take the bread out of the pan 10 minutes after the steam comes out and let it finish baking out of the pan.

Make sure you get the gluten developed by doing 30 stretch and folds 4 times, 30 minutes apart and then do 2 more sets of 4 stretches and folds -  45 minutes apart.   Let bulk ferment on the counter for 1-2 hours in an oiled, plastic covered bowl after the last stretch and fold.  Then pre-shape and then final shape 10 minutes later.  Then pan the dough and let it proof in a plastic bag till it rises 1 inch above the rim of pan.  Watch the dough - not the clock.  Your new starter is still very weak.  

Then in the heat it goes.  That should take care of your problems.

Happy baking  

different types and blends behave differently and produce different results. 

Also, it might help people to know what to tell you if you state your entire recipe and method.