How do I solve this error code in relation to this script for unity? [closed]

You as can see in the List<T> documentation, it resides in System.Collections.Generic namespace.

Therefore you can solve the error by adding:

using System.Collections.Generic;

(this is mentioned by the compiler: "are you missing a using directive ...").

Alternatively (although I wouldn't recommend it here), you can use the fully qualified name whenever you mention a List, e.g.:

public System.Collections.Generic.List<GameObject> propPrefabs;