When a user tries to create a new resource, it returns “Failed to add new resource over the hard credit”. Why??

This means Plan Credit Limit has been reached. User must have run out of money on his balance (if he pays by check), or his credit card failed to be charged (expired/was broken, etc.) In this case the user can still run his account (with recurent and extra usage payment acrued making up his negative balance), but creating new resources (both paid and free) is disabled.