Is Mac OS X Server 10.6 upgradeable to latest Desktop OS X (without Sever.app)
Once all the updates have been installed and OS X 10.6.8 is installed the App Store suggest to upgrade to OS X 10.11.6.

Therefore, it plays no role whether it is an OS X servers or a "normal" OS X.
As soon as it is 10.6.8, it is eligible to upgrade.