May I know what is the error that occur when opening the Quartus?
Is the Quartus not starting? Perhaps you can refer to this video for troubleshoot.
You can check the prerequisite for the ubuntu 18.04 before installing:
Perhaps the installation is not done properly. Some anti-virus scanner can prevent the Installer from downloading and installing software files to your machine. Recommend that you disable any anti-virus or malware scanner before the download and installation start.