There was a bug in the 7.1.2 version of the install script. I think you have that version. (See: Virtualmin install script version 7.1.2 released - #2 by Joe)
Unfortunately, it’s a pretty nasty bug that puts the spamassassin package into an unconfigured state that is annoying to get out of. I recommend starting over with a fresh OS install and the 7.1.0 version of the script.
If you don’t have the 7.1.2 version of the installer script, then it’s a different problem, and we need to see more of the log, probably. (But, this is where the 7.1.2 version fails, so I think that’s what you have.)