Your identity synchronization from on-premises is unhealthy
When you try to run the Synchronization Service Manager on the server where you installed the Azure AD connect it gives you the below error
Unable to connect to the Synchronization Service
Check the Microsoft Azure AD Sync service and restart it, if you are unable to restart it -> Restart your server and check if it automatically starts
If still the service is not started, double click on the service and click on Log On — Put the AAD service account username and password and now try to start the service, it should start now.
Now open PowerShell as administrator and run the command
Start-ADSyncSyncCycle -PolicyType Delta
Now open the Synchronization Service Manager to check the Sync status
Check the Azure AD Sync status on Microsoft Admin Center
Your Azure AD Connect started syncing now :)