Maximum Street Cred Level is 50.
You get Street Cred for doing blue Quests, those related to the NCPD.
These include the blue Baseball Bats, Blue Question Marks and Blue Skulls.
You will only need to do a fraction of all available ones, I only cleared the North-East Outskirts and reached 50 easily. I was Level 27 when this popped. 15 of those levels I gained by doing these, just going from one to the next.