I'm making progress, your suggestion did solve the "Enter Parameter Value" issue.
The next thing that is confusing me, is that when I run the query, I have almost three times as many rows as before. I'll just explain again what I'm try to achieve, because I deleted my post before.
So, I have grouped by player ID and engagement ID in a query, to get a sum per player per engagement. There are 363 results, and that seems to be correct.
When I enter this formula - Expr1: IIf([T_MusiciansDetails]![VAT_Number]<>"",[T_Payments]![Amount]*0.2,"") I get 922 results. I'm not sure why that is yet.
Edit...Bob, I deleted my post literally one minute after posting. arnelgp was so quick to reply that I didn't notice nor expect anyone to reply in that time. But I get your point though, I won't in future.