Since this plugin is a paid plugin, we require permission from the plugin developer to add it to Minehut. Sadly, the plugin developer isn't interested in allowing us to add this plugin. Sorry about that!
This is a plugin API that's used by other plugins. Unless there's a different plugin that requires this plugin, you wouldn't be able to use it alongside command blocks.
Can confirm this is a secret message. To get to it requires figuring out the few ciphers this message went through. I’d recommend checking the plugin info as well as the plugins API endpoint as it all includes info you will need to decrypt this secret message.
PS: This is really almost impossible with the little amounts of info you have so I can give you some hints over time to help you out.
Minehut is not using the version you are linking. The version you linked is one we used in the past but it didn't work so we switched to the one I linked in this post that's embedded (https://www.spigotmc.org/resources/groupmanager-1-7-1-14.40615/). Seems like they had an update July 8th so I can try updating to that one but if that doesn't fix the issue, I really don't know what else to tell you. The plugin hasn't been maintained by the original author in years now so I would highly recommend using LuckPerms over GroupManager.
This plugin has not been updated in over 7 years now. It is way too outdated and will most likely not work on the current version of Minecraft that MInehut is running on.
Like stated above, this plugin only supports the Minecraft version 1.8 and Minehut servers are currently using version 1.14.3, meaning this will not be compatible with Minehut.
It doesn't seem like this Skript addon is being updated anymore, it hasn't been updated in almost 2 years now so not sure if it will work well on 1.14.3