The HTTP 401 Unauthorized for /apm/info is possibly caused because of the webserver before the Mendix application is blocking the url. If the application is running on-premise the webserver need to allow the url.
For more info on a IIS webserver see: (5.4 Configuring the URL Rewrite)
In Mendix Cloud v3, you need to allow the request handler /apm.
To check if the browseragent is loaded correctly open the browser developer tools and take a look at the Network tab.
If the browser agent is loaded successfully and placed on a page, snippet or layout that a user will open in the application this will be visible on the APM Dashboard.