Hello
The issue may be relate 2 things
1. Check connection from vCenter to DNS server
2. If connection between vCenter & DNS server is OK, next step checking about DNS service on vCenter (can login to VCSA:5480), ssh to vCenter for check about configuration, cat /etc/hosts for config FQDN, cat /etc/resolv.conf for DNS IP server.
3. Final try reboot vCenter
Regards,
Duy