To add such calendar you need to add the account it belongs to in iOS Settings:
1. Launch iOS Settings app .
2. Go to Passwords & Accounts and tap Add Account. If you're using iOS 10.3.3 or earlier, go to Settings > Mail, Contacts, Calendars > Accounts and tap Add Account.
3. Select option you need and sign in with your account credentials.
4. Make sure that Calendars toggle is enabled in the account settings.
5. Launch Calendars 5 and tap .
6. Then tap and go to Local Calendars settings menu and make sure that the toggle is enabled there.