PPTP is very old and has known security weaknesses. Apple have quite rightly killed it off. You can think of PPTP as being so weak security wise as to be equivalent to the ancient WiFi encryption scheme called WEP which also no-one should be using these days.
Server.app 5.2 is the oldest version that will run under macOS Sierra. If you want to run an older version you must first downgrade to El Capitan.
Even L2TP is known to also have security weaknesses although not as bad as PPTP. I suggest you configure L2TP in Server.app if it is not already done so - including setting a Pre-Shared-Key and tell your users the appropriate settings.
Note: You can use Profile Manager to push settings out to users.
Frankly I personally regard Apple's VPN server as a bit of a joke and not fit for business use - only home use. I setup a Linux based Cisco IPSec compatible VPN server with security certificates and VPN on Demand and routing all traffic via the VPN which is far more secure. The built-in Mac VPN client can still connect to this Linux VPN server.