Migrating AD from SBS2008 to server 2016 issue

That's some sort of replication issue.  Have you run DCDiag or checked the logs in Event Viewer?

To be very clear, have you checked using active directory users and computers on the SBS server?  Is it the SBS dashboard that isn't showing users? Or is it ADUC?

Not showing in the AUDC not on the dashboard. The intention is to demote the SBS , since the first replication from SBS to server 2016 seams to be good is it save to shutdown SBS after move the FSMO roles?

Can you share the replication health report? you use ADRep tool from Microsoft

Elevated CMD on both:

RepAdmin /viewlist *
RepAdmin /SyncAll
RepAdmin /KCC 

Select allOpen in new window

Please post the results into a CODE snippet so they are easier to read.

Are you using the same OUs on the new 2016 server as the ones on the SBS server?  IOW, are you using the replicated OUs under "MyBusiness" on the 2016 server to add the new user(s)?  Have you tried moving the users out of the SBS MyBusiness OU into a separate new OU on the 2016 server?  That's normally what I do, since the replication of the SBS OU's simply (IMO) complicates that organizational unit tree and they are no longer needed on your Windows Server 2016.