For all that extension ask for an expressvpn app install in the browser extension, this solution i have really works, also in Opera browser:

If you installed your web browser from the Ubuntu Software Center, you may find that the ExpressVPN browser extension is unable to communicate with the ExpressVPN Linux app. To resolve this issue:

Back up your browser settings and bookmarks.
Go to the Ubuntu Software Center and uninstall your browser.

Open the Terminal.

Enter:
$ sudo apt update

Depending on your browser, enter:

$ sudo apt install chromium-browser

or:
$ sudo apt install firefox

Open your browser by entering:

$ chromium-browser

or:
$ firefox

Get the ExpressVPN browser extension by entering:

expressvpn install-firefox-extension

or:
expressvpn install-chrome-extension

ENJOY