The Supervisor was removed from vCenter, but the VCF Automation 9.0 did not clear it out of the Region it was associated with.
Log into the VCF Automation and under Infrastructure select Regions. In the Supervisors column of the Region may contain an old Supervisor name, or may display an extra comma indicating there should be something displayed previous to the comma.
Edit the Region and check the Supervisors selection box. At the bottom of the box it should indicate the number of Supervisors next to a check box on the left, and on the right it will say "# - # of # Supervisors" indicating the number of Supervisors it knows of. If the number is larger than the number of displayed Supervisors available to select there is a stale entry.
VCF Automation 9.0
A Supervisor server was removed from the vCenter environment, but the VCF Automation Region still has a record of it that didn't get cleaned up.
Issue is expected to be resolved in the 9.1 release of VCF where user can edit the region to remove stale supervisor and add new one.