@JenJen
If you installed OBS via flatpak you will need to install the addon via flatpak as well. The default download on the linked page is a deb package which would work with OBS installed from the mint apt repos or from source.

The flathub page for OBS has install links which should trigger a flatpak install if you select the addon from the list near the bottom of the page.
https://flathub.org/en/apps/com.obsproject.Studio

You can also check the list of installed flatpak packages with the terminal command `flatpak list` and install the addon from the terminal with `flatpak install flathub com.obsproject.Studio.Plugin.AitumStreamSuite`.