Plesk support has a help page on that: https://support.plesk.com/hc/en-us/articles/12377562126743-MySQL-fails-to-start-when-a-Plesk-server-lacks-disk-space-SQLSTATE-HY000-No-space-left-on-device-500-Plesk-Exception-Database

Delete temporary files that are older than 14 days... 

# find /tmp -type f -mtime +14 -exec rm {} \;
# find /var/tmp -type f -mtime +14 -exec rm {} \;

# rm -rf /usr/local/psa/PMM/tmp/* /usr/local/psa/tmp/*

Check the size and clean package cache...

# du -sh /var/cache/apt/
# sudo apt-get clean

To remove the oldest Plesk backup file...

# /usr/local/psa/admin/bin/pmm-ras --get-dump-list --type=server | grep 'message' | grep -v [0-9]_[0-9]
backup_info_2010230005.xml: 
backup_info_2010300005.xml: 

The first one in the output is the oldest one. Remove it using its name...

# /usr/local/psa/admin/bin/pmm-ras --verbose --debug --delete-dump --dump-specification=backup_info_2010230005.xml --session-path=/var/log/plesk/PMM

Find all files bigger than 200 MB size...

# find / -type f -size +200M -exec du -h {} + 2>/dev/null | sort -r -h
248M /var/log/plesk-php74-fpm/error.log
218M /var/lib/mysql/ibdata1

There will be a log files among them like error.log in above example, you may clean them up...

# echo > /var/log/plesk-php74-fpm/error.log
# service plesk-php74-fpm restart

Restart services...

# service psa restart