Dist-upgrade from Ubuntu 14 to Ubuntu 16 was performed;
Plesk Repair Kit is not available at https://203.0.113.2:8443/repair/, it shows error 404;
plesk-repair-kitpackage is not installed:
# dpkg -l | grep repair
It is a Plesk bug with ID #PPPM-10927. It will be fixed in the next Plesk updates.
Until the fix become available, connect to the server via SSH, run
plesk installer and, without selecting anything, run it.