Stan has been away for some time, and I understand he may now be back.
Yes, I'm back living under my rock again. It's cozy, and there's beer.
I tested contents of clipboard by cmd v on a separate page. Tested correctly.
I've never used a Mac, so I can't say that this would work. The terminal is different from other applications though. Normally on a non-Mac computer, CTRL+V will paste into a document, but that doesn't work in my terminal... and CTRL+SHIFT+V is needed, as
@Vrai mentioned above. And he also said above to "use the mouse"... but "right-click and paste" also does not usually work in terminal. But sometimes it does work to "middle-click" (push down on scroll wheel) to paste.... or you can click on the Edit Menu at the top of the terminal screen and choose Paste from there. Of course you must have successfully copied the activation code first, in any case.
@Vrai also suggested typing in the activation code from the keyboard, and you may need to resort to that, but I know it is a long and difficult string of characters... be careful about "0 and O" and "1 and l," if you try this.
So I went to my account and got an activation code, copied that to my clipboard, went to terminal and typed in activate expressvpn. Pressed enter, was than asked to enter activation code, pasted that into the required field ( nothing showed in screen but was definitely in my clipboard, ) pressed enter and nothing happened.
I put "activate expressvpn" in bold type because that command is wrong. I suspect you actually typed the command properly, which is "expressvpn activate" because it responded to you properly asking for the activation code. When you paste it the activation code, it is normal that you do not see anything, and you just hit enter, as you did. If nothing happened, I would guess that you are not pasting from the clipboard correctly.
The short 3-minute video referenced by
@f33dm3bits shows the activation steps exactly. Although you are using Linux Mint, you would still choose Ubuntu 64-bit version of ExpressVPN... unless by chance you installed 32-bit Linux Mint, then you would need the 32-bit Ubuntu version. Things like that should match, but again, you should be fine because you had no complaints about the installation steps.
I just let my subscription to ExpressVPN expire, so I can't test or verify anything with it anymore. But it is a good service and I don't have any complaints about it (other than the cost). Everyone above gave good advice, and you should be able to follow the steps from the video, and from ExpressVPN technical support people. If not, you have a 30-day window to cancel them if you can't make it work.
Good luck!