The type ‘string’ must be a non-nullable type in order to use it as parameter T in the generic type or method ‘System.Nullable’

Use string instead of string? in all places in your code.

The Nullable<T> type requires that T is a non-nullable value type, for example int or DateTime. Reference types like string can already be null. There would be no point in allowing things like Nullable<string> so it is disallowed.

Also if you are using C# 3.0 or later you can simplify your code by using auto-implemented properties:

public class WordAndMeaning
{
    public string Word { get; set; }
    public string Meaning { get; set; }
}