Click on Connect to QuickBooks button. A new page is open. Log in if you already have an account to authenticate users before you can exchange data with it.  

Or click on Sign up now hyperlink to create a new one: 

After login QuickBooks Online account successfully, click on Authorize button to finish connection process. 

When connecting successfully, the backend page will be displayed as below. Click on Save Config button before continuing. This will help you avoid making the error.