Which weak algorithmic strategies I have that lead me to failing coding this simple algorithm?

I am a graduate student in computer science who graduated recently from a famous university. I applied to my first software engineering job and I failed the technical interview by failing to answer the first extremely silly programming question. I wasted all my time on trying to solve this question and never touched the other questions, yet I failed to answer it. I want to know why I failed and which weak points in my problem-solving thinking I have that I need to address and solve.

First here is the question: given an input n that defines the number of columns, return a string that contains an X. A picture for what I mean:

Here is my stupid approach of solving it:

public static void main(String[] args) {


        int size = 9;

        for ( int i = 1; i <= size; i++) {

            if ( i <= size/2 ) {
                for ( int k = 1; k < i; k++ ) {
                    System.out.print(" ");
                }

                System.out.print("*");

                for(int j = i; j < size-i; j++) {
                    System.out.print(" ");
                }

                System.out.print("*");
            }
            else if ( i == (size/2)+1 ) {
                for ( int k = 1; k < i; k++ ) {
                    System.out.print(" ");
                }
                System.out.println( "*");

            }
            else {

//              for ( int k = 0; k < (size/2) -1;  k++) {
//                  System.out.print(" ");
//              }
//              
//              System.out.print("*");
//              
//              for ( int j = i; j > size-i; j-- ) {
//                  System.out.print(" ");
//              }
//              
//              System.out.print("*");

            }

            System.out.println();
        }

As you can see, although I wasted 1 hour just trying to answer it, I haven't even completed the solution. The next day I went to two friends of mine and they solved the questions in 2 minutes by creating a 2D array of chars and using a single for-loop. That shocked me and made me feel that I am in fact a very stupid person. This actually affected me personally since having a graduate degree in computer science from a famous university and not being able to solve such a ridiculous question. I took too much time for my graduate studies so I became seriously thinking that I have a mental problem or something :/

But anyway I decided to go with a positive attitude and try to solve my problem. So based on the kind of that programming question and my solution to it, here are my questions to you:

1- Given the approach I used to solve the problem, am I fixable somehow regarding my problem-solving approaches?

2- If so, what kind of problem-solving/algorithmic category does that kind of question fall into? & how to improve?

I need to note that during my studies I had to code many complicated algorithms and software.