Please let me know following.
1. Did you use this plugin before starting this issue?
2. Do you have messages (messages between users) from previous version?
3. Check database and see if there any database table name start with PREFIX_fep_ where PREFIX if your database table prefix. if yes, let me know how many and what are they.
4. Do your set database user in wp-config.php have permission to create database table?
It seems for any reason DB table was not created.
Please open functions.php of this plugin and in line around 28 change version_compare( $installed_ver, FEP_DB_VERSION, '!=' ) (change != to =) and refresh your website. Check your DB is those tables are created. Then change it back as it was.