Object key cannot be edited

What causes this issue?

You branched to the key maintenance screen. However, maintenance of
keys is not possible, because the first field of the key is not
internal data type C. In this particular case, the entire object must
be transported.

System Response

Maintenance is not allowed. Continuing to the key field screen is
possible only for entries already in existence.

How to fix this error?

Transport the entire object. We recommend that you define key fields
with the internal data types C, D, N, or T as these types cannot be
represented in compressed form and consequently unrestricted
maintenance is possible.

Error message extract from SAP system. Copyright SAP SE.