No plugin option on my server's aternos page

So I want to add a plugin to enable skins in my server, the problem is that my server has no plugin option in its page. How can I enable a plugin that will add skins to my server?

Server is using pixelmon generations and Minecraft 1.12.2

When I try to install the software it says that it will delete my world. Is there any way to install the plugin without deleting my world?