QuickBooks Error 3120 often signifies a problem with the connection between QuickBooks and your bank. This error can be triggered by issues such as incorrect online banking credentials, corrupted files, or outdated software. To resolve Error 3120, verify your login details, update QuickBooks to the latest version, and check your bank connection settings. For further assistance, you can contact QuickBooks support at 1-844-248-9739 for expert help and guidance.