Aria Operations for Logs 8.18
The Management > Certificates UI tab is to upload certificates into the trust store, not for replacing the certificate of the Web UI or API itself.
Use the Configuration > SSL tab to upload the certificate desired to secure the Web UI and API.
Follow the instructions in Upload Signed Certificate.
See: VMware Aria Operations for Logs 8.18 - SSL Certificate Replacement for more details on how the certificate can be replaced in Operations for Logs 8.18.
See: Installed custom certificate shows as unsecure connection when navigating to the VIP in Aria Operations for Logs for details on using a certificate on the VIP of a load balancer.