This is not how you reference custom folders, you're supposed to use the folder ID. Only "known", built-in folders can be referred to by their name, as explained for example here: https://learn.microsoft.com/en-us/graph/api/resources/mailfolder?view=graph-rest-1.0 For any other folder, you should use the Folder ID. You can list the set of folders to get their IDs, or search/filter for the folder in question:
GET https://graph.microsoft.com/beta/me/mailFolders?$filter=displayName eq 'MVP DL'
GET https://graph.microsoft.com/beta/me/mailFolders/AAMkAGU2MWM5NzU0LWY3MmQtNGI3OS1hNDVlLTBkMzI3OWVlNWIzZgAuAAAAAAChKSJAhlnUTIHtKSso30ThAQBIPfDMxyP-RYhY8M8xmAPVAACPwDEQAAA=/messages