In Dynamics GP both payables and receivables transactions have the following rule:
You can only apply credit documents to debit documents. Not the other way around. Because of this, the apply button will be disabled on any debit documents.
In Receivables:
Debit documents: Invoice, Debit Memo, Finance Charge, Service / Repair
Credit documents: Credit Memo, Return, Payment
In Payables:
Debit documents: Invoice, Finance Charge, Misc Charge
Credit documents: Return, Credit Memo, Payment