Hi there! It sounds like you're having trouble applying for entries in Dynamics GP because one of your vendor ledger entries is marked as "not open". While it's not possible to apply entries to closed entries, there are a few things you can try to open up the entry and make it available for application.
First, check to make sure that the entry is not marked as a historical transaction. If it is, you may need to remove the historical flag in order to make it available for application.
If the entry is not marked as historical, you may be able to open it up by reversing the credit memo and then re-entering it as a new transaction. This should create a new, open entry that can be applied to other transactions.
Another option is to manually adjust the vendor's account balance to reflect the credit memo, rather than applying it directly to another transaction. This can be done using a manual payment or credit note, which will create a new, open transaction that can be applied to future transactions.