You have encountered an ‘Oh No’ pop window instead of the regular Friend Convert tool.
Friend Convert works only when you are logged into your Facebook account. Therefore, you need to log in to your Facebook account, go to your intended group of which you are a member, then go to the ‘Members’ tab of the group. Then you click on the Friend Convert icon from your chrome extensions bar to operate the tool.
For a detailed step-by-step illustrated guide for a smooth start, please click here.