Trying to complete the "Create an approval process that validates when a Prospect Account becomes a Customer."

Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Account number must be 8 characters long.: [AccountNumber]

 

Hello,

This is the trailhead questions which I am trying to solve :

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.


Here is my code :

public class StringArrayTest {
    public static List <String> generateStringArray (Integer n) {
       List<String> List1 = new List<String> ();
        for(Integer i=0;i<n;i++) {
          List1.add('\'Test'+ i+'\'' );
  }
        System.debug(List1);
        return List1;
    } 

}


I am getting following 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.


I tried it many times but I am not able to solve this problem. Please help.