Apologies, I should've given clearer instructions how to easily open the terminal.
The SUID sandbox helper binary was found, but is not configured correctly.
Thanks! This sounds familiar, it's an Electron application. Essentially a web application running on the local computer.
To fix, run these commands in the terminal:
sudo chown root /home/user/FoundryVTT/chrome-sandbox
and:
sudo chmod 4755 /home/user/FoundryVTT/chrome-sandbox
(The * won't appear when typing your user password for sudo)
Your program should then launch when you double click it and click "Run".
Apparently, this happens because the program is using an older way of handling this sandbox [1]. Those commands satisfy its requirements by saying "root owns this chrome-sandbox executable; and when executed, run as the owner (root user)". Another Electron app had the same problem, not sure if it's due to newer Ubuntu versions.