[Server thread/ERROR]: [ModernPluginLoadingStrategy] Could not load plugin 'AutoPickup v1.2.jar' in folder 'plugins\.paper-remapped'
org.bukkit.plugin.InvalidPluginException: Cannot find main class `net.jadedmc.autopickup.AutoPickup'
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:82) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at io.papermc.paper.plugin.provider.type.spigot.SpigotPluginProvider.createInstance(SpigotPluginProvider.java:125) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at io.papermc.paper.plugin.provider.type.spigot.SpigotPluginProvider.createInstance(SpigotPluginProvider.java:35) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at io.papermc.paper.plugin.entrypoint.strategy.modern.ModernPluginLoadingStrategy.loadProviders(ModernPluginLoadingStrategy.java:116) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:38) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:40) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:561) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:329) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1230) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:333) ~[purpur-1.21.1.jar:1.21.1-2327-4b57bed]
at java.base/java.lang.Thread.run(Unknown Source) ~[?:?]
Caused by: java.lang.ClassNotFoundException: net.jadedmc.autopickup.AutoPickup
at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:199) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:166) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at java.base/java.lang.ClassLoader.loadClass(Unknown Source) ~[?:?]
at java.base/java.lang.Class.forName0(Native Method) ~[?:?]
at java.base/java.lang.Class.forName(Unknown Source) ~[?:?]
at java.base/java.lang.Class.forName(Unknown Source) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:80) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
... 10 more

Good plugin, but it will be better, if you create placeholder, that show last picked up item