The name 'j' does not exist in the current context

int[,] arr = new int[5, 5];
            int counter = 0;

            for (int i = 0; i < 5; i++)
            {
                for (int j = 0; j < 5; j++)
                {
                    arr[i, j] = (j == 0) ? 0 : counter;
                    ++counter;
                }
            }

            for (int i = 0; i < 5; i++)
            {
                for (int j = 0; j < 5; j++)
                {
                    Console.Write($"{arr[i, j]} ");
                }
                Console.Write("\n");
            }

Semicolons in C# terminate a statement, so this one:

for (int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++);
                               ^
                               |
    Console.Write($"{arr[i,j]} ");
    Console.Write("\n");

ends the loop, and does not include any other lines in it. If you look at your code you will see that the indentation reflects that: the Console.Write instruction is not indented at all from the for loop above it.

As a result, the variable jdeclared in the loop goes out of scope immediately and is not available in the next line.

This may help you next time you get a compilation error: How to Write Code to Solve a Problem, A Beginner's Guide Part 2: Syntax Errors[^]