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I'm here to share the details about opening your QuickBooks Desktop backup file, ChongFF.

Currently, the desktop version of QuickBooks is available in US, Canada, and the UK.

Based on your description of the issue, it seems you bought the QuickBooks Pro 2020 version in one of the regions mentioned. If you have the US version of QuickBooks and your back up file is created using the UK one, that's usually when the error occurs. This is because they're different in some aspects and you're unable to use a file that's not for that country version.

To fix this, I'd suggest buying and installing the QuickBooks Desktop version with the respective region of your backup company file.

Here's how to install:

1. Open the file you downloaded (QuickBooks.exe).
2. Follow the onscreen instructions.
3. Accept the software license agreement, then select Next.
4. Enter your Product and License numbers. Then select Next.

For the step-by-step guide, you can refer to this US article about installing the software: Install QuickBooks Desktop. It also contains the hyperlink to successfully activate the program in the Step 4: Activate QuickBooks Desktop section.

Moreover, here are some of the US articles that'll guide you in successfully managing your business using QuickBooks Desktop: Get QuickBooks Desktop user guides.

Please let me know if you have other concerns. I'm just around to help.

@ChongFF 

Your old data file is created on a Non US version (e.q UK, CA, AU). You have to purchase a 3rd party conversion service.