解答例として

Please find below a little fix for your issue.

The problem was the space between Test and the number you had : Test0, Test1.... and what was necessary was Test 0, Test 1,

public class StringArrayTest {
    
    public static List<String> generateStringArray(Integer n)
    {
        List<String> myArray = new List<String>();
        for(Integer i=0;i<n;i++)
        {
           myArray.add('Test '+i);
           System.debug(myArray[i]);
        }
        return myArray;
    }

https://developer.salesforce.com/forums/?id=906F0000000AvPBIA0

Please add ';' at the end of the code.

StringArrayTest.generateStringArray(3);

I DONT UNDERTAND HOW RESOLVE THIS POINT

List<String> stList = StringArrayTest.generateStringArray(3);