Using Linux Mint had the same Problem and fixed it that way.
Most problems with Linux and the new launcherapp is, that the app reqires openjava 11 , all modpack require openjava 8.
Fist uninstall the ftplauncher.
Install both versions of Openjava.
Reinstall the FTBlauncher.
Within the Launcher choose "Java Edition"
Goto "Installation"
Choose the version you want to play
Goto Edit
Goto Extendet Options
There set the path to java8.
Save your settings
The modpack should be startable now
Where are you seeing these?