Reinstalling Mac mini (server?)
From your photo, it looks like the disk has two Recovery Partitions for different updates of 10.11 (El Capitan). 
It also looks to have an installer disk partition: like you would make for a USB stick in order to boot and install a version of MacOS.

I would select the first Recovery Partition (10.11.4), and from there launch Disk Utility and merge the other two partitions. Then install OS X from there.

If you've already tried installing from the Recovery Partition, and that's not working, then you could try Internet Recovery, which is what you do when the disk doesn't have a valid Recovery Partition. Option Command R.

Then try the same process from there: re-partition the disk and install the OS. It may install a different OS, but once it's done that, you can upgrade to whatever you like.