You actually need to download the official 2Checkout Payment Module in order for it to work. Here’s the link to 2Checkout’s Payment Module for WP Invoice: https://github.com/craigchristenson/2checkout-wp-invoice
To install this just download the zip file, unzip contents and upload the entire folder within your wordpress directory under “plugins” by logging into your web hosting FTP or C-Panel.
The directory pathway is wordpress => wp-content => plugins
