It has been a while and finally I am back! I have D365O VM on my machine and I have been fiddling with in the environment for a little bit now. I attempted to install mPOS using D365O. The process was much smoother compared to the previous version of AX 2012 R3. There were less parts that we had to worry about and made the installation of mPOS a breeze. I was able to get everything setup and configured within an hour or so. Everything went great until I tried to login to mPOS. I would get an error relating to external identity. I remembered that I forgot to associate myself to a worker for the store I'm using. However, during the setup I ran into an issue with associating external identity. I was unable to find my email that I need to associate myself to one of the worker that belongs to the retail store. With a little bit of research I found out there was another way to associate yourself to the worker.
1) Go to worker 000160 (usually the worker we mainly used in the VM).
2) In "External Identity" action pane click "Clear Identity".
3) In the worker select the "Retail" tab and click "Edit".
4) In "External Identity" section fill the following textbox with this information.
These information can be found if you login to Azure Portal.
Alias: email of the AAD User (Azure Active Directory)
UPN: Email of the AAD User
External sub identifier: AAD's User's Object ID
5) Once you're done you should now be associate to worker 000160, and should be able to login to mPOS without any error.
Have fun exploring!