Error - 'Cannot change Invoice Register approval details.' when editing Invoice Register Options

This error indicates that you have some Purchase Invoice Register entries remaining in the invoice register batch.
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Before you can edit either the PL Approval or POP Approval settings in Invoice Register Options you have to release all the batched records that are currently awaiting processing, approval or posting.
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This can be done by either Approving and posting through any invoice register entries or deleting entries from the invoice register.