AJournal will copy the calendar/reminders access info to iPhone. But the user needs to authenticate it again in AJournal settings.
Please open AJournal settings on your iPhone and tap the Calendar and Reminders again; AJournal should show a popup asking the user to allow the app to access the calendar and Reminders on your iPhone.
If the user uses AJournal on multiple devices, please ensure the calendar names match exactly. If the name does not match, please rename it in the iOS Calendar app.