In vCenter UI, an alert might be shown at the top of the screen.
There are expired or expiring licence in your inventory.
In vCenter UI, Menu > Administration > License > Assets > Solutions tabs shows instances in the Evaluation mode.
vCenter Server 8.x
Licenses added for the assets will be expiring in few days. Hence, the warning reported in vCenter UI.
Add the license which is having the longer expiry dates to the assets to fix this issue.