After installing PsychoPy from scratch I also had the PTB priority error on openSUSE Leap 15.2. As far as I understand from the error message, Psychopy is not run with the correct real-time permissions / task priority when started as a normal user. Following the above instructions, I first created the group “psychtoolbox” and added the user to this group:
sudo groupadd --force psychtoolbox
sudo usermod -a -G psychtoolbox <username>
Then I added a new file called “99-psychtoolboxlimits.conf” in the directory “/etc/security/limits.d/” (which was empty). The content of the file was as above:
@psychtoolbox - nice -20
@psychtoolbox - rtprio 50
@psychtoolbox - memlock unlimited
After a reboot PsychoPy worked nicely, maybe this helps for cross platform installations.