wrong windows version. check your windows version (it's either pro, home, home single language, x86 or x64) and download the correct iso.
if you have upgraded before (run setup.exe in windows), just skip places asking you for key.

the upgrade path has been highlighted many times in this thread. i have no clue why people are still getting it all wrong lol

1. you need to upgrade the activated os first. connect the computer with activated os to the internet, download the iso with the correct windows version for the activated os with media creation tool. extract the iso contents in a folder or just create a bootable usb out of the iso. run the setup.exe in the folder or bootable usb in normal windows environment (not booting it from usb) to do upgrade (the computer must be connected to the internet during the upgrade);
2. once you have completed the upgrade, connect to internet again to make sure the new w10 is activated. that's all for the upgrade.
3. if you wanna do clean install, do step 1 and 2 above first, then boot the computer with the bootable usb instead of windows. you can wipe the os drive clean, and then proceed. when asked for key, skip it and proceed. after clean install, connect the computer with new w10 to the internet and it will be activated automatically. no key is required.

note that for the free upgrade offer, you only need to run step 1 and 2 once (the upgrade), then you can do clean installs as many times as you like, without w10 key, for the lifetime of the device.
i think your windows version is single language x64 bit.
wrong. read my reply above.