Prefix number: entry missing for system & client &

What causes this issue?

Tasks, rules, and workflow definitions require an ID that is unique
throughout all systems and clients. In this way, you can ensure that
you can transport these objects from one system into another at any
time (without restrictions). From a technical point of view, the
uniqueness of the ID is ensured by what is known as a "prefix number".
You can define a separate prefix number for every system and every
client in table T78NR.

System Response

If a prefix number is not defined in the client in which you are
currently working, it is not possible to maintain (maintenance
terminates).

How to fix this error?

Maintain table T78NR in <DS:SIMG.SIMG_WF15OOW4>Customizing</>, and
create an entry for the system in question and the current client.

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.