Installing on MS Windows
For installation of the Invoices plugin on Windows, you need to install the Bitnami Redmine stack first. Please check this guide which will show you how to download and install the Bitnami and then the steps for installing the plugin via Bitnami.
Note: Please note that before installing the Invoices plugin, you have to install the Contacts (CRM) plugin first because it is required for Invoices to be able to work. You can use a Free (Light) version, which would be enough for the proper functioning of this plugin.