Is Starting wolfram-workbench when Mathematica already running not allowed?

Question

There is similar error reported in this question wolfram-workbench-doesnt-launch-mathematica I am getting same problem. If I add another comment there, it will not be seen.

So I am posting this here so it can get attention. Adding yet another comment "I am also getting same error" will be ignored as there is already comment there to same effect and two years has passed with no answer.

I just started using Wolfram workbench and still learning how to use it.

This is the problem. When I open just one notebook in Mathematica before I launch workbench, it open ok.

Now I launch workbench OK. I can edit the .m files no problem. But within the workbench if I try to do "debug as"->"Wolfram" to launch the notebook associated with the project, or just normal launch (i.e. not in debug mode), like this

It fails to start the notebook. It gives

Here is full screen shot

It is asking me to activate Mathematica !

This only shows up if I had opened a Mathematica notebook before starting workbench. When closing Mathematica itself, running outside workbench, the error goes away.

Is this a known limitation? Does this mean one has to close Mathematica itself before starting workbench?

This is strange, since I am able to open two instances of Mathematica on my PC with no problem.

Mathematica 13.1 on windows 10 pro.

Eclipse IDE for Java Developers (includes Incubating components) Version: 2022-09 (4.25.0) Build id: 20220908-1902

Using latest Wolfram workbench.

Update. Additional information

I have zero for $KernelCount. My Mathematica is student version 13.1.

this says in help

$KernelCount gives the number of subkernels available  for parallel computations.

I am not doing any parallel computations? But if this is the cause, that is fine. I will know that I need to close all my open notebooks when I use Workbench. I just find this a bit annoying, because it means I am not able to do anything else in Mathematica while debugging.

Update

I found same problem happens as follows. Close everything. Start workbench, and launch notebook within the workbench for debugging as screen shot shows above. Everything is OK so far. Now I went and started Mathematica itself. It will not start. It asks me to activate. Same problem.

Another problem. Close everything. Start workbench. and launch notebook within the workbench for debugging as screen shot shows above. Everything is OK so far. Now I am not able to even create new notebook using File->New notebook (from the notebook already open in workbench) and have it run using new kernel in order to be able to run commands.

I get error saying limit is reached even though I only have one single notebook open.

All of this means I am not able to issue any Mathematica command anywhere while debugging. This is big problem for me. The debugger is nice, but I want to be able to also issue Mathematica commands in order to examine some things while debugging.