This error message is happening because your QuickBooks account subscription has expired.
To fix this error, please follow these steps:
Sign in to your QuickBooks Online account
Click on the "gear" (settings) icon in the upper right of the screen, then select "Account and Settings". Then click on "Billing & Subscription". There you can enter or update your billing information.
Go to ContractorTools and select "QuickBooks Sync" in the main menu.
Tap on "Sync Errors". This will take you to a list of the sync errors.
For each error, tap on it, then tap Clear.
Now go back to the QuickBooks Sync screen and tap "Sync Now".