The My Shift Planner app requires a certain amount of storage capacity in your phone to work properly. If you get below 100MB of storage space on your phone, the app won’t be able to save shift changes and will trigger a pop up message to warn you.
For the app to work properly, you’ll need to clear some space for it. Such as deleting old photographs, messages or unused apps.
This link HERE has some great tips for finding that extra space on an iPhone.