[14:47:23 INFO]: Fyrlex issued server command: /pa checkpoint 1 Parkour [14:47:23 ERROR]: null org.bukkit.command.CommandException: Unhandled exception executing command 'pa' in plugin Parkour v4.5 at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:181) ~[spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at org.bukkit.craftbukkit.v1_7_R4.CraftServer.dispatchCommand(CraftServer.java:767) ~[spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.PlayerConnection.handleCommand(PlayerConnection.java:1043) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.PlayerConnection.a(PlayerConnection.java:880) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.PacketPlayInChat.a(PacketPlayInChat.java:28) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.PacketPlayInChat.handle(PacketPlayInChat.java:65) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.NetworkManager.a(NetworkManager.java:186) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.ServerConnection.c(ServerConnection.java:81) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.MinecraftServer.v(MinecraftServer.java:734) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.DedicatedServer.v(DedicatedServer.java:289) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.MinecraftServer.u(MinecraftServer.java:584) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java:490) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:628) [spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] Caused by: java.lang.NoSuchFieldError: STONE_SLAB2 at me.A5H73Y.Parkour.Utilities.Utils.isCheckpointSafe(Utils.java:891) ~[?:?] at me.A5H73Y.Parkour.Course.CheckpointMethods.createCheckpoint(CheckpointMethods.java:74) ~[?:?] at me.A5H73Y.Parkour.ParkourCommands.onCommand(ParkourCommands.java:89) ~[?:?] at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot-1.7.10-SNAPSHOT-b1657.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95] ... 13 more

Although updating the server would fix a problem, you could change the property "Other.EnforceSafeCheckpoints" to false in the config.yml, then enter "/pa reload" and this will fix this error.
For the next update I will add a fallback that will disable this feature once an error like this is detected.

Use older version of Parkour plugin. Server is too old (1.7.9!) to run it