FarmVille 2's request envelope requires third-party cookies to function properly. There are many extensions / add-ons that may be blocking these.

If you are using Firefox, you can find out if your browser is blocking third-party cookies and data by following the steps listed below:

1.) Access the Firefox menu  in the upper right corner of your browser window, then click Add-ons and themes.

2.) Go to the Extensions tab in the left navigation pane, and “Manage Your Extensions” will show the extensions currently installed in your browser.

3.) Note any extensions that may be blocking cookies (for example: "Privacy Badger", "Cookie Monster", etc.). In the 3-dot Menu, you can check the function of each extension by clicking on the Options button or Remove button to remove the extensions that block third-party cookies. You can also toggle  each extension on/off.

4.) Reload FarmVille 2 and check if the envelope is working.