If you open the folder that has the server jar file, then go into the plugins folder.. do you have another "plugins" folder inside that one? It sounds like you do. If so, move everything in the second plugins folder back to the first one.
There should not be a folder path like (server folder)\plugins\plugins lol.
Interesting, because the error shows 'plugins\world...' in the name of the plugin that can't load and when I deliberately made a sub plugins folder this is what I got:
Strahans-MacBook-Pro-1260:Minecraft strahan$ java -Xmx1G -Xms1G -jar spigot-1.11.2.jar
Loading libraries, please wait...
[16:35:58 INFO]: Starting minecraft server version 1.11.2
[16:35:58 INFO]: Loading properties
[16:35:58 INFO]: Default game type: SURVIVAL
[16:35:58 INFO]: This server is running CraftBukkit version git-Spigot-b32c8f8-4d3bf20 (MC: 1.11.2) (Implementing API version 1.11.2-R0.1-SNAPSHOT)
[16:35:58 INFO]: Debug logging is disabled
[16:35:58 INFO]: Server Ping Player Sample Count: 12
[16:35:58 INFO]: Using 4 threads for Netty based IO
[16:35:58 INFO]: Generating keypair
[16:35:58 INFO]: Starting Minecraft server on *:25565
[16:35:58 INFO]: Using default channel type
[16:35:58 INFO]: Set PluginClassLoader as parallel capable
[16:35:58 ERROR]: Could not load 'plugins/worldguard-6.1.jar' in folder 'plugins'
Looks familiar, eh? Open a terminal and go to the minecraft server folder, then go into plugins. Do ls -l, do you see a plugins folder in that list?