Keep getting "type mismatch in expression" Error

[ewan97]

I've been trying to get a query to run but I keep the "type mismatch in expression" error message.
It's the Invoice-Product Query in the attached file
I have tried changing the field types, the primary keys and messed around with the relationships but noting seems to work

(all info in the database is made up)

 

[MarkK]

The fields on which you are doing your join are not the same type. One is a long integer, one is a string.

 

[ewan97]

I fixed the problem by starting again from scratch. My first attempt was too far gone to put any more effort into.
I made sure the linked fields where formatted the same way this time as well.