Redirects: “Not all the required fields are filled.”

I want to start creating redirects, as I am moving from our old site to a new site in WordPress. After I have created all the redirects, I am going to point the IP for the old www. to the new site which doesn’t use www, and I will be using Yoast SEO to redirect some of our important pages. (The rest I will allow to go to 404.)

So, I have my list ready, but now, when I start adding data, I get a problem. There are three fields in Redirects: Type, Old URL and URL.

I select the 301 for Type, I enter my old URL and my new one, but when I click to save, I get this error:
“Not all the required fields are filled.”

Under Redirect Settings, I have the defaults enabled, viz.
Redirect method: PHP
Generate a separate redirect file: Disabled

Where are these ‘required fields’ which I still need to fill in?