The desktop app version didn’t fully support all extensions unless prebuilt and installabe via pip or conad, see here. Jupytext should qualify for that.
However, jupyter nbextension install - isn’t going to do anything for your desktop JupyterLab. That is for the classic notebook interface running in a browser. Not JupyterLab and not the desktop app.
That line conda install jupytext -c conda-forge should probably have worked though if installing to the right place.
If you try running !jupytext --help in a notebook, does that seem to do anything? That would indicate if some aspects of jupytext installed because you should see the man page.