Sour dough using unbleached all purpose flour is very wet during the preshaping
(I am assuming your starter is at 100% hydration, i.e., that it is half water, half flour. I am also assuming you have no/little experience in baking bread. Please correct me if wrong.)

Looking at your ratios, you have a total of 460g water (400g as water, 50g in the starter, 10g with the salt) to a total of 550g flour (including 50g from the starter). This gives you an overall hydration of just over 83%, which is definitely a wetter dough. Handling wet dough is not easy. I think there might not be anything wrong with your dough itself (although see below), and you just need to get more experience working with wetter doughs.

One thing that may make the dough wetter than the recipe assumes: both bread flour and (especially) whole wheat flour absorb more water than AP flour. Substituting AP flour for the bread flour is not going to make a huge difference, but still might make the dough wetter than is expected.

My advice: lower the hydration to somewhere in the 65%-70% range (i.e., replace the 400g water with 297g-325g). Learn to handle this dough (this might take a few trials), then start increasing the hydration.