If you went to the trouble of buying a Purism laptop for privacy/security, I have no idea why you’d want to install Flash on it.
Adobe Flash is one of the most security-compromising things out there, which is why it’s being completely phased-out. Please see this from the Chromium blog: https://blog.chromium.org/2017/07/so-long-and-thanks-for-all-flash.html
The internet has moved on to HTML5. If I were you I’d start boycotting sites that still use Flash. They need to get into the modern age or fade into irrelevancy.
No solutions for your problem, just trying to talk you out of getting Flash. I think the new standards for the internet should be “Everything in SSL and HTML5 only”. I go as far as setting HTTPS everywhere to block all unencrypted requests, essentially boycotting all sites that don’t use SSL.
That’d make a lot of tabloid newspapers and the likes unavailable to you, but thankfully I don’t read that garbage anyway. Seriously - people that are still using non-SSL and Flash probably shouldn’t have your business because it shows their incompetence and unwillingness to put in the work on it’s own.
I’m just saying, it’s rather silly to have dropped so much money on this and expect everything to be the same as running a Windows computer with Chrome installed, which it kind of feels like that’s exactly what you did.
Anyway, if your problem is that sudo apt-get isn’t working, you could try downloading from here and running from the file I suppose: https://get.adobe.com/flashplayer/otherversions/
I’m not currently a Linux user so I can’t really help you beyond that.