No one has actually pointed out that the problem is a syntax error in the if statement.
A single
=
means assignment, as in
int i = 0;
In order to compare equality you need to use double
==
So you could correct your code by simply adding a second
=
if (ddl_Moratorium.SelectedItem.Text == "NONE")
txt_Morat_Mths.Enabled = false;
else
txt_Morat_Mths.Enabled = true;
As has been pointed out, this is not safe as you will get a NullReference error if ddl_Moratorium.SelectedItem is null.
Solution 5 gives you the most readable code.
You can also change your code to a one liner, but it is of course less readable. :)
txt_Morat_Mths.Enabled = ((ddl_Moratorium.SelectedItem != null) && (ddl_Moratorium.SelectedItem != "NONE"));