If you are unable to add a partner user despite having available user licenses, it is likely due to one of two reasons:
Partner License Limit: The specific partner company you are trying to add a user to has reached the limit set in the "Max licenses they can add" field for that partner. You must edit this field to increase the limit.
License Type Discrepancy (Cost): If your organization is being charged a different rate for regular user licenses versus partner licenses, the system may prevent an available, lower-cost regular license from being automatically consumed by a partner user. This issue requires intervention from the Finance and Customer Success teams to resolve your specific billing configuration.