I had this problem recently on a Punto.
Check if the brake lights work. A faulty brake switch could cause this problem.
If they light up normally, check for wire damage of the exposed part of the wheel sensor cables.
Check the battery voltage as already mentioned.
If you find nothing abnormal, get a Fiat specific OBD tester and read the faults.
In my case, it started with an eroded wheel sensor cable (eaten by the tyre) which I soldered twice as some part of the copper had already oxydized by water ingress. The fault went away for some time, after months it came back, only this time it was caused by a faulty brake light switch which was a bit cumbersome to replace.