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My Mac operates with Sierra and my version is 10.12.6 (16G2136).
I can t upgrade.
With your OS you can use Illustrator CC 2018 (version 23).
The installation file(s) you can download here: Adobe CC 2018 Direct Download Links: Creative Cloud 2018 Release | ProDesignTools
I have a suggestion but you won't like it.
Buy a Newer (doesn't have to a brand new) Mac or switch to a Windows system that doesn't have the Apple Restriction on the age of your computer to run a modern operating system.