QuickBooks Error Code 12, 0 is typically related to issues with the network connection or firewall settings. Here are some steps you can take to resolve this error:

Update QuickBooks: Make sure you are using the latest version of QuickBooks. Check for any available updates and install them.

Restart your computer: Sometimes a simple restart can resolve temporary issues. Restart your computer and try opening QuickBooks again.

Disable antivirus and firewall temporarily: Temporarily disable your antivirus software and firewall to check if they are causing the error. If the error is resolved after disabling them, you may need to adjust their settings to allow QuickBooks through.

Run QuickBooks File Doctor: QuickBooks File Doctor is a built-in tool that can help resolve various QuickBooks errors. Run the tool and let it scan and repair any issues it detects.

Repair QuickBooks installation: If the error persists, you can try repairing your QuickBooks installation. Go to the Control Panel, select "Programs and Features" (or "Add or Remove Programs"), locate QuickBooks in the list, and choose the option to repair the installation.

Check network connectivity: Ensure that your computer is connected to the network properly. If you are using a wireless connection, try connecting via Ethernet cable to rule out any issues with the Wi-Fi connection.

Check firewall settings: Make sure that QuickBooks is allowed through your firewall. If necessary, configure the firewall settings to allow QuickBooks access to the network.

Create a new company file: If the error persists, create a new company file and try opening it in QuickBooks. If the new file opens without any issues, it may indicate that the previous company file is damaged and needs to be repaired.

Regards,
Peter