GC,
The message is self explanatory; I don't know how it could be stated much more clearly. The actual steps you take will depend upon what you have currently assigned to this resource.
Open the resource in //RES01 in Change mode. Select the resource; select the Definitions button. Delete assignments of unneeded objects that are currently assigned to your resource. Select the Capacity Profile button. Reject all Changes to Bucket profiles. Since you are evidently starting from scratch (completely re-defining this resource), you may also wish to consider deleting the assignment of any capacity variants in the General Data Tab.
Determining the Valid Available Capacity - Resource - SAP Library
Best Regards,
DB49