QuickBooks Error 3120 occurs when there is a data mismatch between QuickBooks and an external application, typically during transactions. This error usually arises from issues with invoice or sales order numbers. To resolve it, verify and correct any discrepancies in the transaction details, ensuring all relevant data fields match between QuickBooks and the connected app. If you encounter any issue, contact the QB Support team, dial
1.855.409.2300.
