A trace has already been activated for user & in ST01

What causes this issue?

The trace can only be activated for one user or group of users on each
application server.
Since a trace has already been activated for the specified user in ST01,
you cannot activate any of the performance traces (SQL trace, RFC trace,
HTTP trace, enqueue trace, buffer trace) on this application server.

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?

Wait until the other user deactivates the trace. You can then activate
the performance trace and other traces.
You can also activate the trace for all users, or for users whose names
match a pattern. In this case, specify the pattern in the usual format.
You can activate the trace for multiple users by choosing the button
"Activate Trace with Filter".

Error message extract from SAP system. Copyright SAP SE.