A case can't be deleted if any financial matter postings have been done, even if they are been reversed off and put on the correct matter.
The matter/case will need to be archived following the normal steps.
If this case is part of your GDPR and needs deleting after so many years there are specific GDPR programs that can be added on the Toolbar.
Also the Case and Accounts Deletion Program can be used for these items.
If you have any queries please raise a new case online and reference the title of this article.