32 bit and 64 bit compatability

So I am working on a project for my father-in-law's business and I just sent what I have right now to him to test it and he came up with an error saying that it needs to be updated to function with 64bit. I have put over 40 hours into this worksheet and I don't want to lose it all. What kind of changes would I need to make to make it compatible?

Here is the spreadsheet if that helps. (Some of my macros aren't complete yet either so just ignore those ones.) Thanks for any help you can provide.
Inventory Management Accounting.xlsm