That version of server does not appear available via the App store unless you had purchased it in the past with your Apple ID you can redownload your past purchases.
You can Buy OS X Lion(10.7.x):
http://www.apple.com/shop/product/D6106Z/A/os-x-lion
You can Buy OS X Mountain Lion (10.8.x):
http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion
Mavericks (10.9.x), Yosemite(10.10.x), El Capitan(10.11.x)
Were free upgrades, however if you did not download or add them to your purchase history when they were originally released they are not available.
Sierra(10.12.x)
Is available on the Mac App Store as a free download as it is the current release at this time. I would suggest adding it to your purchase history even if you do not plan on installing it.
Upgrade and migrate data on your macOS Server - Apple Support
I did notice I can download an older version of Server on my 10.10.5 Machine and I can upgrade to the latest release but any versions before that I have not seen the ability to do so.