You may configure staffing resource capacities based on the number of people of your group sales reservation. For example, for every 15 adults/children/seniors etc., you may require 1 volunteer or staffing resource. The error will occur if there are not enough resources for the size of the group.

To resolve the issue:

Increase the quantity for the staffing resources

OR

Set the Capacity per resource to "0" and attempt to enter any quantity for the staffing resource again