This error is happening because ContractorTools is attempting to write a "transaction" to QuickBooks that is dated before the end of the last period closed in QuickBooks.
A "transaction" can be either an Estimate, Invoice, or a Payment.
To fix this error, you have two options:
1) Go into QuickBooks and remove or change the last closing date, to a date after the transaction you are trying to write to QuickBooks, or
2) Change the date of the ContractorTools transaction to a date later than the last closing date in QuickBooks.
Once you have done either of these, then:
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. Or tap "Clear All Errors".
Now go back to the QuickBooks Sync screen and tap "Sync Now".