Captcha - How to insert into form?

Captcha - How to insert into form?

I am building a website contect form. It generally works fine.

But when trying to place a captcha I cannot select the item "captcha" neither can I select "file upload". Both are located in the extended tools section, whereas all items from the field list ("Name", "Surname" etc.) work though.

What's wrong?
Tnx.