Base amount missing for withholding tax amount entered

What causes this issue?

For a withholding tax category/combination, you entered a withholding
tax amount but not the base amount.

System Response

The data is incomplete and no processing is possible.

How to fix this error?

Enter the missing base amount or delete the withholding tax amount. If
you do the latter, the system automatically determines the amount and
base amount.

Error message extract from SAP system. Copyright SAP SE.