Process &1 still active in &2 of the canceled client copies of &3 &4

What causes this issue?

This client contains at least one active process from a cancelled
client copy.

System Response

You cannot perform a new client copy or a restart until all processes
end.

How to fix this error?

Wait until these processes have finished their current tasks, or cancel
them, e.g. with the transaction SM66 (although not during a database
I/O).

Error message extract from SAP system. Copyright SAP SE.