Conversion of & from POOL to TRANSP unsuccessful

What causes this issue?

The table type could not be changed from pool to transparent. The table
probably violates one of the restrictions for transparent tables. For
example, only 16 key fields are permitted for a transparent table.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.