Target group &1 is undefined or empty

What causes this issue?

You have chosen the target group &V1& as the transport target of your
request. The target group does not exist, or it does not contain any
elementary transport targets. Therefore, it cannot be used as a
transport target.

System Response

The input is rejected.

How to fix this error?

Choose another transport target.

Procedure for System Administrators

You can create and maintain target groups in the Transport Management
System (transaction STMS).

Error message extract from SAP system. Copyright SAP SE.