I see www-apps/radicale-3.1.8 marked as testing, but not masked.  The place to look for masking reasons is /usr/portage/profiles/package.mask (or wherever your portage tree lives.)

However, if I search for radicale, I only see the one package, and the associated acct-group and acct-user, so I don't know if you just used * to quote the name, or if there are related packages I'm not seeing.

Thank you.

How do you see that radicale is marked for testing?

I don't have /usr/portage/profiles/package.mask on my system, but I found a second package.mask in

        /var/db/repos/gentoo/profiles/package.mask

but it doesn't mention radicale.

Are you saying that if a package is "masked", portage is only aware of that fact because it's specified in a file on my system?  But it's also shown as "masked" here:

https://packages.gentoo.org/packages/www-apps/radicale.

The actual error mesg:

      !!! All ebuilds that could satisfy "radicale" have been masked.
      !!! One of the following masked packages is required to complete your request:
      - www-apps/radicale-3.1.7::gentoo (masked by: ~amd64 keyword)
      - www-apps/radicale-3.1.5::gentoo (masked by: ~amd64 keyword)

The asterisks were supplied by thunderbird (or something thereafter) to implement the "Bold" font I specified.