I was solving the challenge of trailhead of Apex development:

The question was:
Create an Apex class that returns an array (or list) of formatted strings ('Test 0', 'Test 1', ...). The length of the array is determined by an integer parameter.The Apex class must be called 'StringArrayTest' and be in the public scope.
The Apex class must have a public static method called 'generateStringArray'.
The 'generateStringArray' method must return an array (or list) of strings. Each string must have a value in the format 'Test n' where n is the index of the current string in the array. The number of returned strings is specified by the integer parameter to the 'generateStringArray' method.

My code is:


public class StringArrayTest {

    static List<String> TestArray= new List<String>();
    public static String[] generateStringArray(Integer n){
        
        for(Integer i=1;i<=n;i++){   
            TestArray.add('Test'+i);
        }
        
        return TestArray;
    }
    
}

I am able to get desired outpuut but still it is returning an error:
Challenge not yet complete... here's what's wrong: 
Executing the 'generateStringArray' method failed. Either the method does not exist, is not static, or does not return the proper number of strings.

Not able to understand what it want exactly