Question 22 is a calculation, and 21 is a transfer from 28, so my guess is you're asking about 23: "...enter the portion of basis attributable to section 263a costs."
Good question! Short answer: leave it blank or put 0 here unless you know what it is.
Longer answer: https://www.plantemoran.com/explore-our-thinking/insight/2020/01/tax-inventory-accounting-final-regulations-bring-changes (You can read about it!)
Summary: this relates to inventory accounting, and specifically how you are valuing your inventory. For the small business taxpayer the simplest solution is often not to depreciate your inventory. If you do depreciate it, there are rules you are supposed to follow. This is one of those rules :-).