What air pressure is needed on mars, to have fluid water?

Here is the phase diagram for water from Wikipedia:

The average Martian surface temperature is 218K. From the diagram, you can see that at that temperature, there cannot be liquid water, however, you specified 277.15K. From the Clausius-Clapeyron relation, the required pressure for liquid water to form at 277.15K is 812.7 Pa. Given that the pressure on Mars ranges from 30 Pa to 1155 Pa (on Olympus Mons and in the Hellas Planitia respectively), and that the temperature on Mars can get as high as 300K in the summer, it is certainly possible to have liquid water on Mars temporarily at certain times of the Martian year.

The triple point of water is at 611.73 Pa and 273.16 K (0.01 °C), so it's actually very close. There can be liquid water in the lowest laying regions, especially if it comes as brine, but it won't last long and the temperature has to be just right.