There are 2 mains reasons
Shift is Pending
A shift needs to in booked state and pending shifts cannot be invoiced. See Shift States for details.
Rate is missing for Client within Shift
A shift will marked as red in Client Shift Breakdown if no rate (rate group) has been attached to Client when shift was saved. This means that this particular can only be invoiced once correct rate (rate group) has been attached to shift.
You can fix this by clicking on update attendance icon for client
Note! Please note that shift can also be red if there is nothing to invoice i.e. hours in shift is 0 and program/additional cost along with mileage is also 0.