I've set up my domain on my host correctly following the steps. Have my correct server IP and when trying to save the A name to point to my host - I get this error. I tried this a few days ago and had the same issue. I was wondering if someone could give advice? I'm using Cloudways as the host and they pointed me here for support as they're insisting the IP address is fine and everything is set up their end correctly.
I notice in your screenshot you already have a zone record setup for 'www' as a CNAME that points to the root IP '@'. Your likely encountering this error cause 'www' already exists.
Try changing the Host field to '@' instead as you don't appear to have your root record that allows the domain without 'www' to resolve to the site.