Certificate Manager will not work if vCenter Server is in High Availability mode (VCHA). If yes, you will have to follow below approach :
Break VCHA -> Replace Certificates -> Reconfigure VCHA again
If VCHA is not at all configured, more investigation needed to identify the actual cause. Please check the availability of file /etc/vmware-vcha/vcha.cfg, renaming it might help to proceed further (Please DO NOT perform the same if VCHA is already configured)
Let us know if any additional questions.
Thanks