Two things must be toggled on in order for calendar sync to function, one within MyCue app, and the other in your iPhone Sertings:
Within MyCue app, go to Profile screen,
1. Tap on Edit
2. Go all the way down and make sure Calendar sync is toggled ON
From your iPhone, go to your Settings,
1. Tap on your name all the way on the top
2. Tap on iCloud
3. Under the section for APPS USING ICLOUD, make sure Calendars is toggled ON