In order to check in for shifts, you must enable location settings for the Z app.
To enable locations settings for the Z app follow the instructions below:
Quick Links:
Apple iPhone
Tap on the settings app on your phone 
Scroll down to the bottom of the screen and tap on the Z app. (Its probably at the bottom of the list).
Next tap on the Location option
Then tap on "While Using the App" or "Always" option
Android