Debugging an external session is not allowed (profile value: &1)

What causes this issue?

Debugging an external session is not allowed because the profile
parameter 'abap/ext_debugging_possible' is set to <(>&<)>1.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.