QuickBooks can be set up to download transactions from your bank account. After QuickBooks downloads transactions, you match the downloaded transactions with transactions you’ve already entered in QuickBooks, or instruct QuickBooks to create a new transaction. The "match download transactions" process assigns downloaded transactions to an account in QuickBooks.
Payments are the only type of transactions that ContractorTools writes to QuickBooks that are also downloaded from your bank account, so if you're getting this error, it is related to a payment (which will be referenced in the error message in ContractorTools).
The error message is saying that the ContractorTools-Quickbooks sync process is attempting to update a payment transaction that has already been matched to a downloaded payment in QuickBooks, and there is a problem because the payment was assigned to a QuickBooks account when it was downloaded, but the Quickbooks sync process is attempting to change that account.
This is most likely because ContractorTools writes payments to the "Undeposited Funds" account in QuickBooks, but when you match downloaded deposit transactions in QuickBooks, they get moved to your "bank account" account in QuickBooks (to indicate that the payment has been deposited into your bank account).
So the most likely reason this error is happening is because you entered a payment in ContractorTools, synced it with QuickBooks, then downloaded and matched a bank deposit transaction to it, but then something was done to change the payment in ContractorTools (changed the date or the payment amount), so now ContractorTools is trying to update that payment in QuickBooks.
In any case, please follow these steps to fix this error:
Delete the payment in ContractorTools.
Run the QuickBooks sync in ContractorTools again.
Verify that the payment transaction was pulled from QuickBooks into ContractorTools.