Volunteer Report Error: Expression #2 of SELECT list is not in GROUP BY clause
As you must be aware CiviVolunteer is not fully tested for 5.5.x version of CiviCRM. The last extension version was released for 5.0. This error usually occur when sql_mode for mysql is configured for only_full_group_by. I have just submitted a pull request for this fix.

You can either apply the patch from here or turn off only_full_group_by sql mode

HTH

Pradeep