Google currently requires the initial “From URL” subscription to be added in Google Calendar on the web, not inside the Android app. After you add it once to your Google account, it will sync into the Google Calendar app on your Android phone.
On a computer, open Google Calendar.
Next to Other calendars, click +.
Choose From URL.
Paste the calendar URL below and click Add calendar.
In Outlook on the web, choose Add calendar → Subscribe from web and paste the link below.
Your calendar app controls refresh timing; changes may take time to appear. If you saved a one-time copy before, replace it with a subscription.