Google Calendar only supports calendar URLs up to 250 characters. If the calendar feed URL exceeds 250 characters, users will need to run the URL through a shortener in order to add the feed to Google Calendar

In order to shorten a calendar feed URL:
  1. Copy the URL of your calendar feed
  2. Go to https://bitly.com/
  3. Paste the URL of your calendar in the box
  4. Remove the webcal:// section from the beginning of the URL
  5. Click Shorten 
  6. Copy the generated short URL
  7. Add the shortened URL to your Google Calendar
Another option is to filter the event categories included in the feed to reduce the length of the URL