Upgrading my old iMac from Yosemite to High Sierra

Hi,

I have an old iMac I'm trying to upgrade.

It's a 3.06GHz Intel Core i3 from mid 2010 and it's on Yosemite.

It's supposed to take OS up to and including High Sierra.

I can't log in to apple store. I get an error, but it shows up on my phone with a 6-digit number.

However the mac doesn't give me the option to write that number. It's too old I guess.

I have tried a lot of things. I've tried to create a new apple ID (not possible), I have tried to make a bootable USB drive, but get errors.

Any advice out there?:)

[Edited by Moderator]