[Edi]

More details, please. What map, what number of provinces, what game parameters, have you modified the map file etc etc?

I have never seen that error myself, but I expect it could be caused if you have a command in the map file that specifies a neighbor with a province number greater than what is on the map.

For example, if the map has 100 provinces and you had something like

#neighbour 80 101

which would make province #80 border province #101 (which does not exist). That would presumably cause the error.
In any case, it is generally advised to give detailed, accurate information when asking for help with a specific problem, because otherwise you will get no answers. Vague reports with no detail in them will get you nowhere.