How do i buy Licence for an older version?

I have an old MacBook Pro (Early 2011) which is still going strong but will not update from 10.13 (High Sierra). I’ve been using Logic for a long time but I’m interested in giving Cubase another go. I’ve downloaded a version of Cubase Pro 10.5 (the last supported version) but need an activation key. Do I purchase a key for Cubase 11 to activate it or do I need a specific key for 10.5?
Any help appreciated.

A Cubase Pro 11 activation should allow you to run previous versions (Pro 10.5).

When you say “another go”, do you mean you already have an older Cubase version license? If so, there are discounts for upgrades/updates.