Have you been able to install the mods after enabling administrator rights? You can try to enable that permission permanently following these steps: You can do that by right-clicking in the Nexus Mod Manager shortcut, and there choose the option Run as Administrator. The easy way to fix this issue is by running NNM with administrator rights.
This could happen because you have an old version uninstalled that left some data or problems with the permissions. The first reason why the mods could be appropriately installed on your games is that it doesn’t have the appropriate permissions to read or modify your files.
Solution 1: Run NNM Installer as Administrator
If you are of those users that have been experiencing this problem, on this post, I will leave you some solutions to fix installation error and be able to start Nexus Mod Manager. Nexus Mod Manager is one of the most accessible programs to manage the mods you installed on your games, but when you face problems during installation, you won’t be able to enable those mods correctly.