that's because you cannot Amend until after 27 Feb.
(ALWAYS, fill in your data wait two weeks to think about it before you file)
The forms to AMEND are "expected" to be ready AFTER 27 February....Don't bother until Mid-March at the very earliest. (I'm serious about that too.).
And don't forget that you will be required to print the forms and mail them in yourself when you are done.
An active link to Amending 2019 will be ready at the following FAQ..(but only when they are ready to allow it)