system project problem
hey everyone! I'm a C# beginner and I'm having some problems on my system code (It's a billing system). Every time I click total, always shows that I need to put a cash amount, idk if this is an window popup error (this also my problem on my login and register password hashing even though I followed it). Does anyone willing to help and teach me how I can fix this? Thanks in advance and have a great day! 
