The review section is not the right place to contact me. The right places would be: sending me a PM, a message via Skype (live:00pflaume), discord private chat (my name is Simonsator and my tag is #5834) or my discord server (
https://discord.gg/pFx9X2W ). I don't get a notification on reviews, so it takes longer until I can help you, if I answer you are unable to answer back to provide me more information, and they are just unfair, as in most cases a user error causes the problem.
You should not install any api. The API should only be installed if you are having the bungeecord edition, but you are having the spigot edition.
Please only install
https://www.spigotmc.org/resources/party-and-friends-extended-for-spigot-supports-1-7-1-15-2.11633 and from this plugin the file "OnlyOneSpigotServer/Clans-Spigot-0.9.9.18.jar" from the zip folder.
As it seems to be a user error and not an error caused by the plugin may I ask you to remove the bad review?