Hi, I am facing error - Ensure that you implement the Queueable interface in the AnnouncementQueueable class. 

I am getting proper response....and implemented trigger, Product2Helper and AnnouncementQueable class as per the trailhead 



Follwoing is my code -
product2Trigger --->
trigger product2Trigger on Product2 (after update) {
   Product2Helper.AfterUpdate((List<Product2>)trigger.new,(List<Product2>)trigger.old);
   
}

Product2Helper Class ---->
public class Product2Helper {

    /**
     * @name COLLABORATION_GROUP
     * @description List of CollaborationGroup used in both business and test logic
    **/
    static List<CollaborationGroup> COLLABORATION_GROUP = [
        SELECT Id
        FROM CollaborationGroup
        WHERE Name = :Constants.INVENTORY_ANNOUNCEMENTS 
        OR Name = :('TEST'+Constants.INVENTORY_ANNOUNCEMENTS)
        LIMIT 1
    ];

    /**
     * @name afterUpdate
     * @description called by product2 Trigger on After Update
     * @param List<Product2> newList
     * @param List<Product2> oldList
    **/
    public static void AfterUpdate(List<Product2> prodList, List<Product2> prodOldList){
        //ToDo: Declare a List of Product2 records named needsAnnouncement

        //ToDo: Declare a Map of Strings to Inventory_Setting__mdt records

        //ToDo: Loop through a query of Inventory_Setting__mdt records and populate the Map with Name as the key

        //ToDo: Loop through the Products in newList
        // Use the corresponding Inventory Setting record to determine the correct Low Quantity Alert
        // If the Product's Quantity Remaining has been changed to less than the Low Quantity Alert
        //      add it to the needsAnnouncement list

        //ToDo: Pass records to the postAlerts method
        List<Product2> needsAnnouncement = new List<Product2>();
        Map<String,Inventory_Setting__mdt> mapInventory = new Map<String,Inventory_Setting__mdt>();
        for(Inventory_Setting__mdt inv : [select id, DeveloperName, Low_Quantity_Alert__c from Inventory_Setting__mdt]){
            mapInventory.put(inv.DeveloperName,inv);
        }
        
        for(integer i=0;i<prodList.size();i++)
        {
            if(mapInventory.get(prodList[i].Family).Low_Quantity_Alert__c > prodList[i].Quantity_Remaining__c && 
                mapInventory.get(prodList[i].Family).Low_Quantity_Alert__c <= prodOldList[i].Quantity_Remaining__c)
            {
                needsAnnouncement.add(prodList[i]);
            }
        }
        PostAlerts(needsAnnouncement);
    }

    /**
     * @name postAlerts
     * @description called by product2 Trigger on After Update
     * @param List<Product2> productList
    **/
    public static void PostAlerts(List<Product2> productList){
        List<ConnectApi.AnnouncementInput> toPost = new List<ConnectApi.AnnouncementInput>();
        for ( Product2 p : productList ){
            // ToDo: Construct a new AnnouncementInput for the Chatter Group so that it:
            // expires in a day
            // does not notify users via email.
            // and has a text body that includes the name of the product followed by the INVENTORY_LEVEL_LOW constant
            ConnectApi.MessageBodyInput msgBody = new ConnectApi.MessageBodyInput();
            ConnectApi.AnnouncementInput tempPost = new ConnectApi.AnnouncementInput();
            ConnectApi.TextSegmentInput textSegment = new ConnectApi.TextSegmentInput();
            textSegment.text = p.Name +'  '+ Constants.INVENTORY_LEVEL_LOW;
            msgBody.messageSegments = new List<ConnectApi.MessageSegmentInput>();
            msgBody.messageSegments.add(textSegment);
            
            tempPost.body = msgBody;
            tempPost.expirationDate = DateTime.Now().AddDays(1);
            tempPost.parentId = COLLABORATION_GROUP[0].id;
            tempPost.sendEmails = false;
            toPost.add(tempPost);
         }
    
        // ToDo: Create and enqueue an instance of the announcementQueuable class with the list of Products
        AnnouncementQueueable annQue = new AnnouncementQueueable(toPost);
        //annQue.toPost = toPost;
        system.enqueueJob(annQue);
    }
}

AnnouncementQueueable Class ---->
/**
 * @name AnnouncementQueueable
 * @description This class posts Chatter Announcements
**/
public class AnnouncementQueueable implements Queueable{

    public List<ConnectApi.AnnouncementInput> toPost;
    
    public AnnouncementQueueable(List<ConnectApi.AnnouncementInput> annList)
    {
        toPost = annList;
    }

    //ToDo: Modify this class to implement the Queueable interface and call the postAnnouncements method
    public void execute(System.QueueableContext context){
        
        PostAnnouncements(toPost);
    }

    /**
     * @name postAnnouncements
     * @description This method is provided for you to facilitate the Super Badge
    **/
    public static void PostAnnouncements(List<ConnectApi.AnnouncementInput> announcements){
        while ( announcements.size() > 0 ){
            if ( Limits.getDMLStatements() < Limits.getLimitDMLStatements() && !test.isRunningTest() ){
                ConnectApi.AnnouncementInput a = announcements.remove(0);
                ConnectApi.Announcements.postAnnouncement('Internal', a);
            } else {
                break;
            }
        }
        if ( announcements.size() > 0 && !test.isRunningTest() ){
            AnnouncementQueueable q = new AnnouncementQueueable(announcements);
            //q.toPost = announcements;
            system.enqueueJob(q);
            //ToDo: Enqueue the above instance of announcementQueueable
        }
    }

}

Any help would be really appreciated.