Community Support team has confirmed that the required field for the Lookup field under Common Data Service can't be selected.
Currently using formulas in each of the PowerApps to accomplish this.
Please activate the Required check box
Look up fields can now be set as required! This is availalbe in the latest update of the Common Data Service.
For information about this update in geneneral, see : https://powerapps.microsoft.com/en-us/blog/cds-for-apps-march/
This work is underway, thanks for your valuable feedback!