Hello Samirkal
The error states the file at /usr/lib/postgresql/9.5/bin/pg_dump is not a valid PostgreSQL dump utility.
Usually, on shared hosting, you won't have access to those system utilities (or anything outside your home / www folder).
Instead, look for backup solutions offered by your host. For example, on cPanel, there are applications to back up (even sometimes automatically) the database: JetBackup, Acronis Backup, etc.
So it is advised to simply remove access to the School > Database backup program on shared hosting (see menu Users > User Profiles).