HI everyone.
I also had problem when I tried install chrome extensions - I got error "crx_file_not_readable".
The problem isn't with extension "Install Chrome Extensions" or Opera program but probably in system.
I did some tests. I installed the same version of Opera (ver. 73) on my PC, laptop and virtual machine (VM). Laptop and VM allows me to install chrome extension using "Install Chrome Extensions" but on PC I couldn't. I tried solution @McSqueezy-0 but it doesn't always work, so I made something around
Opera and Chrome base on Chromium so extensions on both browsers should works same.

1. Find your extensions ex: Dark Reader
   • https://chrome.google.com/webstore/detail/dark-reader/eimadpbcbfnmbkopoojfekhnkhdbieeh
   • Remember extensions id (ex: eimadpbcbfnmbkopoojfekhnkhdbieeh)
2. Install other browser base on Chromium (I used Chrome)
3. Install your extension in other browser.
4. Now you must find where your browser saves extensions. When you do it go that location.
   • For chrome (for Windows) "C:\Users[login_name]\AppData\Local\Google\Chrome\User Data\Default\Extensions" - replace 'login_name' your account name.
5. Find your extension (folder name is the same as your extension id) and copy it (or cut).
6. Go to Opera extensions folder - C:\Users[login_name]\AppData\Roaming\Opera Software\Opera Stable\Extensions - and paste copied folder
7. Open your Opera program go to extensions (Ctrl + Shift + E) and enable "Developer mode"
8. Click "Load unpacked" and go to Opera extensions folder (as in point 6)
9. Find your extensions (exp eimadpbcbfnmbkopoojfekhnkhdbieeh) and go inside the folder.
10. Inside you should see one folder which name equals version of your extension. Select that folder.

Now you should have your extension.