If you bought the NINA template for Joomla 4, there are three folders in the zip file:
You can open the folder: "Demo Installer" and there you will find a completely new JOOMLA! with the sample files from J51. You can install this J51_Nina_Demoinstaller.zip like a normal Joomla. ATTENTION: Check which Joomla version it is after the installation. First update to Joomla 4.2.8 (or higher)!
If you have an existing website, you can install the nina template "empty" from the second folder: "Joomla 4.x Install files". After that you can make all adjustments by hand.
If your website already has content, you would overwrite everything with the demo files!!! So do the following. Simply install the "J51_nina_demoinstaller" in a second installation on your computer (local) or in another folder with a second database! (not the same). Then you can check there in the backend how certain things were solved and recreate it in your installation. (also with this way: Always update to the latest Joomla version first, even with demo installations! Why: See here: https://www.joomla51.com/forum/joomla-4-2-8-release-containing-a-security-fix

But you can also find more information here:
https://www.joomla51.com/tutorials/demo-installer/setting-up-the-demo-installer

I installed joomla in a sub directory or the site, but when I went to the admin screen I received this error.

An error has occurred.

0 Class "Joomla\Plugin\Behaviour\Taggable\Extension\Taggable" not found

here is a possible solution:
https://forum.joomla.de/thread/16446-joomla-4-2-update-fehlerhaft-class-joomla-plugin-behaviour-taggable-extension-ta/

Thanks, that worked.

... funny I had seen that online but others had tried that and it didn't work for them