You're basically creating an error in your "amount in stock" calc field by calling "self" as part of calculation.
Your calculation for amount in stock should just be:
received - used
if you need a starting stock quantity, you need to add a new field, IE "original stock quantity", which would update the calculation for "amount in stock" to be:
original quantity + Received - used
So you know, calculations in filemaker don't require "= answer" on the end. That's treated as part of the calculation, not telling filemaker that is the answer. Your calculation had "= amount in stock" at the end of it, which is the incorrect syntax for what I think you're trying to do.