OK, the version matching is fine. How many APs have been added to the AC6508? The AC6508 supports up to 256 APs by default. If the total number of AP exceeds that limitation, you are required to install a license to expand the limitation.
You can view the online AP through the command ‘display ap all’ to view if the total number of AP has reached the limit. If you are not sure if the license is installed, you can run the command ‘display license’ to review that.