I am not able to download Mojave on my 2010 iMac. Running High Sierra 10.13.6.

It's working properly. Mojave requires an iMac to be a 2012 or newer model.

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brucefromsandwich wrote:

I am not able to upgrade from High Sierra 10.13.6 to Mojave on my 2010 Mac

That's correct. That Mac can run anything later than 10.13.6.