What causes this issue?

For at least one of the tables you want to create a new conversion
program there exist a valid conversion program in phase 'Conversion'.
It is not allowed to create an additional conversion program.

System Response

Process will be canceled.

How to fix this error?

Contact SAP for support.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.