1.) v5.0.6-release-b395
2.) CraftBukkit version git-bukkit-a81f6ed (mc 1.15.2)
3.) There is no option in the config file to enable PlaceHolder api, Multiverse-core/inv and all the other things
4.) https://pastebin.com/KiS3dENC
5.) nope

This is intended, it is automatically enabled when PlaceholderAPI + any other plugin is detected. You will see that it has been successfully enabled on startup.

Thank you