There are a few options to install the Merx plugin.
Merx does not have any special requirements. If Kirby 3 is running fine, Merx also will be fine.
Installation as Git submodule
Run the following command on your website’s root and Merx and it’s dependencies are installed at
git submodule add https://github.com/wagnerwagner/merx.git site/plugins/merx
Read more about Kirby and Git on getkirby.com
Installation via Composer
If you prefer composer you can add Merx with this command from your website’s root. This will install Merx and it’s dependencies at
composer require wagnerwagner/merx
Read more about Composer and Kirby on getkirby.com