Hi,
Thank you for coming Microsoft Q&A forum.
As Windows 7 has end of support since Jan. 2020, we would recommend to upgrade system version to Windows 10.
I tried to access with my test Windows 7 retailed product key, and encounter same error message on that website. So I recommend you feedback this website issue through Feedback Hub application, I also try to feedback on my side. But as you know, Windows 7 has been end of support, so I am not sure if that website is also out of resource or website can't be used any more.
If you have access to VLSC(Volume Licensing Service Center), try to download Windows 7 ISO from there.
If you are using OEM image, try to contact with OEM support to check if they have image for your system.
If any information is useful for you, please accept it as answer which could help other customers find useful information more quickly.
Bests,
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