How to determine Total Float Calculation for Critcal Path

Hi Guys

I am reviewing a contractor P6 programme that has 0 Total Float as it is due to finish before the 'Must Finish By' date. It is therefore showing no Critical Path when scheduled using the Default setting 'Total Float less than 0d'. Can anyone advise what would be best way to determine a Total Float value for the Critical Path calculation? I've tried the longest path setting instead but it's not valid as activities are constrained.

Any advice would be much appreciated.

Thanks