MSN 21 likely refers to a version or iteration of Microsoft's MSN services or software. Historically, MSN has been associated with Microsoft's suite of online services, including MSN Messenger (now known as Skype), MSN Search (now Bing), and others.
If you are on a personal device (like a laptop) using restricted Wi-Fi (like school Wi-Fi), the browser version might be blocked, but the app version might work. msn 21 unblocked