Hi,
From the comment you have added in the other answer it seems that the module you have downloaded is from the following link: Ventor Custom Access Rights
If you check the app store you can see that this module is dependent on an another module, which is : Vendor Base
, So once you download this Vendor custom Access Rights module you will get a downloaded copy of the dependent module Vendor Base. This both modules will be placed inside a folder. So what you have to do is that, rather than placing the merp_custom_access_rights-12.0.1.0.0 in C:\Program Files (x86)\Odoo 12.0\server\odoo\addons place the folders inside the merp_custom_access_rights-12.0.1.0.0 in the above path
Inside the merp_custom_access_rights-12.0.1.0.0 folder you will see two folders named merp_custom_access_rights and
merp_base , copy this folders and place it in this path: C:\Program Files (x86)\Odoo 12.0\server\odoo\addons
Once you done this, restart the service and update the app list by activating the developer mode and check your app in the apps list.
Thanks