Flatpak, snap, and other sandbox systems are designed for isolation-based security. It's intentionally hard to do anything with them that wasn't already done before the package was made, and hard to connect them to the outside world like a media system kinda needs.
Remove the sandboxed version, install the official repository, and install natively from there. For Debian-based systems, which also includes Ubuntu and its derivatives:
Bash:
sudo apt-add-repository ppa:obsproject/obs-studio
sudo apt update
sudo apt install obs-studio
From the sticky thread in this forum:
The Linux version of OBS is still under development and contains many bugs, but if you're feeling brave, and can install Ubuntu PPAs, then you can give this a try. If you're on Ubuntu 14.04 LTS, you'll first need to install FFmpeg: sudo apt-add-repository ppa:kirillshkrogalev/ffmpeg-next sudo...
obsproject.com
Then the normal process to install plugins should also work.