The resource has declined because it is reoccuring you must book every meeting separately with this resource.
This error message is generated by Outlook Direct Booking and is not an error from ERM, even though resources are managed by ERM.
Our recommendation is to toggle the Outlook Direct Booking objects to verify that the Outlook GUI is reflecting the true state of Outlook Direct Booking.
Procedure:
Outlook, Tools, Options, Calendar Options, Resource Scheduling.
Check Automatically accept meeting requests and process cancellations
Check Automatically decline conflicting meeting requests
Check Automatically decline recurring meeting requests
Close Outlook
Repeat procedure, deselecting each object
Close Outlook
Launch Resource Scheduling and verify that all objects are unchecked.
Unmanage and remanage the resource affected
Create a test invitation for the resource, invited as a resource.
*If you continue to get the error message, there are attributes of the resource account that are corrupt and it may require exmerging calendar data, deleting the resource account according to Microsoft recommendations to avoid tombstoning and recreating the account with the original legacy dn and exmerging calendar data to correct.