QuickBooks discontinues QuickBooks Desktop support. Some QuickBooks users that migrate from Desktop to Online account are missing ItemCategories feature which is required for flawless synchronization with RetailTime POS.
With this feature disabled the users will get the following synchronization error in BO: 

Instruction

Please perform the following steps to have ItemCatergories feature enabled for your QuickBooks Online account:

1. Log into your QuickBooks online account.

2. In a new tab within the same browser, navigate to Intuit Accounts - Sign In

3. On the category migration page (example screen attached), click “Switch Now”:
   
   

4. Return to your QBO tab, log out of QBO, close your browser, reopen your browser, then log back in to QBO using your user name and password.

5. Categories should now be enabled.

Now you can synchronize your QuickBooks account with RetailTime.