Action not possible in SAP GUI for HTML. Use external debugging.

What causes this issue?

Direct debugging in SAP GUI for HTML using the classic debugger is no
longer supported.

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?

Use external debugging instead.
To do this, use ADT in Eclipse or SAP GUI for Windows and set external
breakpoints in the places where you want to start debugging. Then
restart the request that you want to debug.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.