Specify a valid pool/cluster name

What causes this issue?

You have selected a physical pool or cluster that does not exist or
was not activated.

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?

Enter a valid pool/cluster name. You can use the ABAP/4 Repository
Information System to find a valid name or else you can create a new
pool or cluster or activate an existing one.

Error message extract from SAP system. Copyright SAP SE.