Console.ReadLine returns a string. You cannot assign that to an integer variable or member of integer array.
First thing would be to look at validating the user input.
Take a look at the int.TryParse method. It will validate the user input, and convert it to an integer, all in the one go
something like
while (int.TryParse(Console.ReadLine(), out liczby[i]) == false)
Console.WriteLine("Try again");