Hello Akimbekov,
Be sure to double-check the serial number with what shows in the Apple menu > About This Mac. This page shows how to check on that: Find the model and serial number of your Mac
If you aren't already, it may help to copy/paste the serial, and check for any spaces before and after the text.
It may also help to try a different web browser, including Safari if you aren't already. This can help to get it isolated down further.
If you continue to have issues, we'd recommend following up with Apple Support directly here:
Get Support