A shift highlighted in green is good news, there’s no action to be taken because shift is in Booked state. If a shift is showing up as red then it means shift is in Pending state.
One of the main reason for pending state is that you have no staff or double booked staff within Pending shift. Click through into Shift Details and then click through staff to checkout their calendar.
Tip! Within shift edit screen, reason for double booking will be present within staff selection dropdown menu aswell.