Ok, I can answer my own question it seems. You need to not only install (in aaPanel) the Mail server, but have it running, which is not trivial, as I had some issues with Rspamd, which are documented:
https://www.aapanel.com/forum/d/4739-rspamd-not-install-please-click-the-fix-button/7
That (including the fix for the external library libsodium) worked. Recap (on Oracle Linux here):
wget -O /etc/yum.repos.d/rspamd.repo https://rspamd.com/rpm-stable/centos-8/rspamd.repo
rpm --import https://rspamd.com/rpm-stable/gpg.key
sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
sudo yum install -y php-cli libsodium
sudo yum install rspamd -y
Now you can install mail server, click submit once you've checked all green in the config checker, then onto php map install....