Thanks for reporting this Neal, I'm spinning up a 2019 server right now to run some tests on.

    I wasn't able to reproduce this using installer defaults (just clicking next). I'm gonna shoot some questions that could help me diagnose what it could be. Was there a specific flow you took through the installer? Did you go back and forth before the IIS page at all? Did you license directly in the installer or did you skip licensing? You mentioned a validation error, did that appear at the bottom of the installer or under one of the fields?

I'm not getting emails from this site for some reason, not sure if it's related to my recent server change but I'm getting other emails...

I downloaded the 7125 installer to Win2019 Standard (Full Installer). Chose the FREE licensing option, I kept clicking next. I do not have SmarterMail in IIS yet as I was going to let the installer do it so I just accepted the values in the create a site for me path. Clicking next flagged an invalid entry as stated in the OP. The inputs were "SmarterMail, empty for hostname, *, 9998"  The validation error was red text at the bottom, none of the inputs showed validation flags to point out where the validation error was. It simply stated the information was invalid or something along that line but didn't point out what was invalid, it was an overall error message on that dialog that it appeared to fail validation of the input group.

Launch installer, FREE edition, Setup a site for me, invalid inputs, I then had to go back and use the create site later option to proceed with the wizard.