It sounds like you may be on VAT cash accounting scheme. If you post a customer receipt as, say, T2, you can only match it against an invoice that is also T2. 

There is some information here: https://gb-kb.sage.com/portal/app/portlets/results/viewsolution.jsp?solutionid=200427112205946

hi there.  i'm having the same issue and normally I just make sure the tax codes are the same and then it works fine.  This is a supplier - I have 2 purchase invoices and 1 credit note - all T5 code as they are an energy supplier.  Go into supplier payment and F3 all 3 lines and the above error comes up.  Any ideas?  I've checked a number of times that the code is the same for all 3.  Is there a work around - I need to show a net payment for my bank reconciliation.  Help!