This occurs when the vendor ledger entry you are trying to apply the purchase credit memo to has a fixed application (a permanent link between entries).
Business Central cannot reapply the credit memo because the existing application must be removed first.
You can try and follow the steps below:- How to Resolve
Go to Application Worksheet: Search → Application Worksheet.
Select the vendor and locate the applied entries.
Unapply the fixed application for the related invoice or payment.
Reapply the entries correctly, then post the purchase credit memo.
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