I realized the plugin wasn't working correctly, and that kits could not be added. I decided to join the support discord and the developer of the plugin told me that there is an issue with the plugin in 1.15, and that Minehut has not updated the plugin to the working version.
It would be greatly appreciated if you could update to the working version, https://www.spigotmc.org/resources/duels.20171/
Thank you!
INCORRECT LINK I AM TRYING TO SORT THIS OUT