The logs say you don’t have the screenshot helper, did you install manually?
Look, I got this screenshot from my machine:
That one is pretty common unfortunately, (I don’t think the method of reading the kwin response from journal is very robust. I plan to switch to dbus in a future release in hopes that will reduce or fix that). But I don’t think that’s the main problem if you don’t have the helper installed.
But it would still fail if you do! On my machine just found another problem:
/usr/bin/kde-material-you-colors-screenshot-helper {36e6a1a4-6ea4-44b4-b087-645787fa3678} out.png Failed to capture window {36e6a1a4-6ea4-44b4-b087-645787fa3678}: The process is not authorized to take a screenshot
KWin is not allowing the process to take the screenshot when is called with the absolute path but calling kde-material-you-colors-screenshot-helper alone works for some reason? I need to investigate why.
If you have the helper now, do you also have the returning non-zero exit status error too?