Hello inaboxrecords, welcome to the Unreal community.
It seems you may have clicked on the Complete Project instead of the plugin. Plugins will generally try to install to the engine. I would search for ‘Metahuman Plugin’ in the Marketplace and obtain it from there. It should look like the page eldany.uy and Ivanim posted a few replies above.
The Metahumans Complete project is only available in 4.26-5.0 so if you were interested in that product, please ensure you have one of those engine versions installed.
I would also make sure no projects are running when attempting to install a plugin.
Lastly, if your engine / launcher needs to be updated, please do so.
I hope this helps.