I have a new server with WHM 8.5.4, cPanel 8.5.4-R72
RedHat Enterprise 3 - WHM X v2.1.1

when I go into CPanel of a user and click on "webmail" it says "Please choose a Web Mail Program:" but it's empty! the icons are not there.

how do I fix that?

Log into WHM
go to Tweak Settings

and turn on the mail programs