[13:22:22] [Server thread/ERROR]: Could not load 'plugins/SkyChanger-Bukkit-3.0.1.jar' in folder 'plugins'
org.bukkit.plugin.InvalidDescriptionException: Invalid plugin.yml
at org.bukkit.plugin.java.JavaPluginLoader.getPluginDescription(JavaPluginLoader.java:167) ~[craftbukkit.jar:git-Paper-62]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:142) ~[craftbukkit.jar:git-Paper-62]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.loadPlugins(CraftServer.java:356) ~[craftbukkit.jar:git-Paper-62]
at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:226) ~[craftbukkit.jar:git-Paper-62]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:884) ~[craftbukkit.jar:git-Paper-62]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_171]
Caused by: java.util.zip.ZipException: error in opening zip file
at java.util.zip.ZipFile.open(Native Method) ~[?:1.8.0_171]
at java.util.zip.ZipFile.(ZipFile.java:225) ~[?:1.8.0_171]
at java.util.zip.ZipFile.(ZipFile.java:155) ~[?:1.8.0_171]
at java.util.jar.JarFile.(JarFile.java:166) ~[?:1.8.0_171]
at java.util.jar.JarFile.(JarFile.java:130) ~[?:1.8.0_171]
at org.bukkit.plugin.java.JavaPluginLoader.getPluginDescription(JavaPluginLoader.java:155) ~[craftbukkit.jar:git-Paper-62]
... 5 more

You need to change the API version to 1.15 in plugin.yml

But how do make it so it keeps the sky? Kinda annoying if you have to run the command every time someone joins the world

I really hate how spigot is deciding my plugin isn't compatible with certain versions, when it undoubtably is. Such a braindead system.

I take that back, the latest version of the plugin is working fine on spigot 1.15.2. I'm not bumping the API version until they force me to otherwise I lose support for older versions.

I suggest filing an issue with Paper as they should not be breaking compatibility if the project they're based off of has not.