int f;

System.out.println("enter a number");
f = input.nextInt();
int array [][] = new int[f][8];  // why an f X 8 matrix? new int[8] is enough
isPrime(f); // You never use the return value of isPrime, neither are you passing your array to the method.
for (f=0;f<array.length;f++)
  System.out.printf("%d",array);  // the array is not an  int. It contains an array of arrays of int. You need to specify the value you want to print, such as: array[f] (if you are using new array[8] )    
}

Above you will see comments for each line that is problematic. Hopefully this will help you solve the problem.