Look in your Dynamics 365 Admin Center, select the instance for your Dynamics 365, then click on Edit. See if there is a Security Group listed in the security group field. If so, then go to Office 365 and add the user to the security group.
Since this is a trial, most folks don't go to this level of security, but it doesn't hurt to check. You can also try removing the license, saving the user account, then re-adding the license. This should kick off the sync again.
Also, be sure you are looking in Dynamics 365 at ALL the users listed. You may be looking at the "Enabled Users" list and the user you are looking for may not be in that list. It doesn't hurt to check to see if all the users are listed.
