There are situations where new users are unable to login into Dynamics 365 for the first time..
Normal way of trouble shooting is to clear the cache and check proper security roles are assigned, proper license is assigned to the user, proper security is assigned in the Office 365 admin area.
Still if the user still unable to login into Dynamics 365 ?
Then Global Administrator/ System administrator login into Admin area and follow below instructions
After investigating, the issue was resolved by selecting the ellipsis, on the custom Dynamics 365 app, and selecting show for all roles, as it was set to hidden for all roles.
I hope this helps..
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