What causes this issue?

You have tried to use the function 'Add additional combinations' to add
entries automatically to the table but these entries do not exist. The
system has determined that new entries exist in the working memory with
the same key as previously deleted entries.

System Response

The system could not enter new entries for the entries with the same
key as deleted entries in the working memory.

How to fix this error?

If you want to enter all the missing combinations, save your data
first. This will clear the working memory of any deleted entries. Then
choose the 'Add additional combinations' function again.

Error message extract from SAP system. Copyright SAP SE.