12.0.18 #77 update failed

Hi , on my Plesk from 1week the update system don't work
this is my log's error messages:
" chown: changing ownership of `/var/lib/psa/dumps/mysql.preupgrade.12.0.18-12.0.18.20160310-034545.dump.gz': Permission denied
tac: write error: Broken pipe"
"
Trying to backup MySQL database...
ERROR while trying to backup MySQL database
Check the error reason(see log file: /var/log/plesk/install/plesk_12.0.18_installation.log), fix and try again"
and today i've a new error on the Home pannel...

12.0.18 #77 update failed