On top of what @rob_marshall pointed out, i.e. using line items to represent time, there seems to be a lot of weird design and model building decisions in your modules and lists.
1. Why does it seem like you have time presented in line items (2019A, 2020A, 2021A) and lists (Fiscal Period List) in the same module, i.e. Sales Trend V4?
2. Why does it seem like you have 2 different lists representing Sales, i.e. GL Name Sales, Gross Sales?
Anyway, to answer your original question, your formula isn't working because you're comparing a number with a list member, i.e. 'Sales Trend V4'.'2019A' is a number, Gross Sales.'2019A' is a list member in 'IF 'Sales Trend V4'.'2019A' = Gross Sales.'2019A' THEN'