Hello, if you've been linked to this page, you're running a Minecraft 1.8 server.
At the time of this page being written, Minecraft 1.8 is 4 years old (released in 2014!).
Servers that far outdated (while still able to run functionally, including with modern plugins) obviously cannot be supported by any reasonable development work. As such, we've prepared this page to help you get your server updated to a supportable version.
First, some information: You can run a more modern server (at time of writing, that is 1.12.2 for stable, or 1.13 for experimental/latest) while still allowing older game clients to join, and enjoying older style PvP if you prefer that.
How To Update Properly
Current Spigot Jar
You will of course need to gather the latest Spigot version using BuildTools: https://www.spigotmc.org/wiki/buildtools/
Letting In Older Clients
To allow older clients to connect, you can make use of a plugin called "ViaRewind", which is an extension of the plugin "ViaBackwards". These are reverse-direction versions of popular plugin "ViaVersion". You can find this plugin here: https://www.spigotmc.org/resources/viarewind.52109/
To enable older style PvP, there are various options available, such as this: https://www.spigotmc.org/resources/oldcombatmechanics-disable-1-9-hit-cooldown.19510/ or this: https://www.spigotmc.org/resources/1-8-pvp-for-1-9-1-10-and-1-11.19291/ or just search the Spigot resources section for "1.8 PvP" and pick your favorite.
Note If You Don't Follow This Guide
If you run a 1.8 Minecraft server, and do not follow this guide to update your server (instead keeping the old version), you can use current versions of the plugins, however you will NOT get any support with making your server work.
Citizens and related plugins (Denizen, Sentinel, etc.) all function fully in their current versions (latest releases and dev versions) on 1.8.8 Spigot servers, however you may not have all features available (primarily those that only work in newer Minecraft versions anyway), and if something goes wrong (such as an error message appearing), there's no guarantee we can do much to help with the issue.
The important thing here is that Minecraft 1.8 is 4 years old at this point and continuing to age, it's nonsense to run a version that old - especially when, as you can see from the information above, you don't need to run that old version on your server to get any of the popular features of 1.8 minecraft.