i bet you did not
and now you can't see what i suggested you to try
you have a DEFIB array of array of strings, so DEFIB[i] is array of strings and DEFIB[i][j] is string
if you receive array of strings as a result of Split method you should assign it to correct data - DEFIB[i], which is array of strings
also, you don't need j loop:
string[,] DEFIB = new string[N,6];
for (int i = 0; i < N; i++)
{
DEFIB[i] = Console.ReadLine().Replace(",", ".").Split(';');
}