My code is given below. It displays error to me:
Error: Compile Error: Method does not exist or incorrect signature: [Schema.DescribeFieldResult].isCreatable() at line 44 column 25
 

public with sharing class CRUD_FLS_Create_Challenge{

    public Id newUser {get;set;}
    
    public List<Personnel__c> getUnReg() {
        unregisteredUsers = new List<Personnel__c>();
       List<Jouster__c> currentParticipants = [ select Participant_Name__r.Name from Jouster__c ];
       List<Personnel__c> currentPersonnel = [ select Id, Name, Favorite_Color__c, Castle__r.Name from Personnel__c limit 15];
       Boolean reg;
       
       System.debug(currentParticipants);
       System.debug(currentPersonnel);
       
       for( Personnel__c p : currentPersonnel)
       {
           reg = false;
           for(Jouster__c j : currentParticipants)
           {
               if(j.Participant_Name__r.Name == p.Name) {
                   reg = true;
                   break;
               }
           }
           
           if(reg == false)
               unregisteredUsers.add(p);
       }
       
       System.debug(unregisteredUsers);
       
       return unregisteredUsers;
    }

    public Jouster__c newParticipant {get;set;}
    public List<Personnel__c> unregisteredUsers;
    
    
    public void register(){
        System.debug(newUser);
        Personnel__c p = [select Name, Favorite_Color__c, Castle__r.Name from Personnel__c where Id =: newUser]; 
        if (!Schema.sObjectType.Jouster__c.fields.Participant_Name__c.isCreatable()) {
            if (!Schema.sObjectType.Jouster__c.fields.Color__c.isCreatable()) {
                if(!Schema.sObjectType.Jouster__c.fields.Favorite_Color__c.isCreatable()) {  
                    if(!Schema.sObjectType.Jouster__c.fields.Castle__c.isCreatable()) {   
                        ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Error: Insufficient Access'));                 
                    }
                }
            }  
        }
        if([select id from Jouster__c where Participant_Name__r.Name =: p.Name] != null)
            insert new Jouster__c(Participant_Name__c=newUser, Color__c=p.Favorite_Color__c, Castle__c=p.Castle__c);
        else
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Error: Jouster already entered in Tournament'));
    }    
    public string[] getPermSets(){
        String[] permSetArray = new List<string>();
        PermSetArray.add('User with Read ONLY Access to the Jousters object'); // description of the needed permission set
        return permSetArray;
    }

}

 

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.