I have this very simple class..  

trigger RestrictContactByName on Contact (before insert, before update) {
    //check contacts prior to insert or update for invalid data
    For (Contact c : Trigger.New) {
        if(c.LastName == 'INVALIDNAME') {   //invalidname is invalid
            c.AddError('The Last Name "'+c.LastName+'" is not allowed for DML');
        }
    }
}

.. and the corresponding Test Class:  

@isTest
private class TestRestrictContactByName {

	@isTest static void metodoTest() {
		
		List listaContatti = new List();
		Contact c1 = new Contact(FirstName='Francesco', LastName='Riggio');
		Contact c2 = new Contact(LastName = 'INVALIDNAME');
		listaContatti.add(c1);
		listaContatti.add(c2);
		
		//insert listaContatti;
		
		// Perform test
        Test.startTest();
        Database.SaveResult [] result = Database.insert(listaContatti, false);
        Test.stopTest(); 
		
		c1.LastName = 'INVALIDNAME';
		update c1;
       		
	}
	
}

When I run the Test class from the Developer Console I get 100% of coverage on the RestrictContactByName class but, when I check the challenge on the trailhead it returns the error:

Challenge not yet complete... here's what's wrong: The 'RestrictContactByName' class did not achieve 100% code coverage via your test methods

Has someone had my same issue?

Create an Apex class that returns an array (or list) of strings: 
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.

MyApexClass to above Challenge:

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;
        
    }

It's compile and execute as per requirement in Developer console. But Traihead showing the below 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.

Anyhelp would be greatly appreciated. Thanks.