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<T>'

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; }
}

string is a reference type, a class. You can only use Nullable<T> or the T? C# syntactic sugar with non-nullable value types such as int and Guid.

In particular, as string is a reference type, an expression of type string can already be null:

string lookMaNoText = null;

System.String (with capital S) is already nullable, you do not need to declare it as such.

(string? myStr) is wrong.