Add external calendar to Google Calendar
I think the Google Calendar app for Android doesn't support that functionality.

From a computer:


I open a Google Calendar, for instance this one
I click on the  button.
That calendar gets added to my Google Calendar.


From an Android phone:


I open a Google Calendar, for instance this one
I click on the  button.
The Google Calendar app opens and displays the message "Calendar couldn't be added".




According to the response from a Google representative, "there isn't an option to add external calendars to the Google Calendar app".