I noticed the same issue with a new server deployment. I'm pretty sure they are non-existent in the current build. That is, unless you source in all of the merc tables manually. I didn't try that so I'm not even sure if that will work.
Instead of doing that, I opted to add bot tables instead and I'm really enjoying them. I still have my old server instance using using my own tweaked merc tables from 2013. Unless there's been recent changes to the way mercs work, the merc tables are completely missing in the current auto server deployment and I don't see an option to add them from the server config menu, only bots.
There might be an updated source somewhere so use at your own risk but the mercs were enabled by sourcing in this SQL from 2013. If this doesn't solve your problem then you'd most likely have to revert to an older build to get them to work.
https://raw.githubusercontent.com/EQ.../svn/mercs.sql