Give access Calendar entries owned by Internal logged-in User.
Configure the user profile on the Object Settings when the tab is not visible.
Go to Setup > Profile > Standard Platform One App User > Object Settings > Internal Use Calendar > Default On > Save.
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Also, give access to the Apex Classes.
Note: You need to configure the Profile of the User that you want to give access to the Internal User Calendar.
In the Standard Platform One App User profile, go to the Apex Class Access and the following Apex Classes
PosiEd.CalendarController
PosiEd.ColorHexGenerator
PosiEd.CustomCalendarPortalHandler
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