Two things must be toggled on in order for calendar sync to function, one within MyCue Business app, and the other in your iPhone/iPad Settings:
Within MyCue Business app, go to Profile screen,
1. Tap on Edit
2. Go all the way down and make sure 'Apple calendar sync' is toggled ON
From your iPhone/iPad 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