Regarding the “Scheduler” button, don’t worry about that - as zabadabadoo noted, it is not a real-time shield, so if the scheduled scan doesn’t allow addition, just run a scan using buttons under “Analysis”.
Regarding updating, set updates to automatic on “Update Assistant”. Some times this feature flakes out due to the underlying ClamAV update having issues. If you notice every day your signatures (under “Update” button) are at zero, then try this in the terminal, sudo systemctl stop clamav-freshclam
then copy https://database.clamav.net/daily.cvd and http://database.clamav.net/main.cvd into /var/lib/clamav (remove any old files)
then again in terminal sudo systemctl start clamav-freshclam