I'm using kdenlive on a custom linux (kernel 5.4.22 based on Devuan). Everythink works very wll, but preview / playback of project monitor sucks. If I switch to vaapi in project, it is a bit better, but it stops every five secons or so. And yes, I cannot activate OpenGL in Playback button of settings.

When I use the (old) 18 release (not AppImage), then OpenGL can be activated and preview / playback in project monitor ist much much faster.

So is ther any way to activate OpenGL in AppImage?

All test done with kdenlive 19.12.3

No. OpenGL is activated in the Kdenlive windows version only.