On a fresh spigot 1.21.10 built with the built tools i get:
Could not load 'plugins\SGReborn-2.5.6.jar'
org.bukkit.plugin.InvalidPluginException: java.lang.NoClassDefFoundError: net/milkbowl/vault/economy/Economy
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:149) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:406) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:313) [spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:122) [spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.craftbukkit.v1_21_R6.CraftServer.loadPlugins(CraftServer.java:470) [spigot-1.21.10-R0.1-SNAPSHOT.jar:4570-Spigot-6e369ba-5c73aaf]
at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:337) [spigot-1.21.10-R0.1-SNAPSHOT.jar:4570-Spigot-6e369ba-5c73aaf]
at net.minecraft.server.MinecraftServer.C(MinecraftServer.java:1063) [spigot-1.21.10-R0.1-SNAPSHOT.jar:4570-Spigot-6e369ba-5c73aaf]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:337) [spigot-1.21.10-R0.1-SNAPSHOT.jar:4570-Spigot-6e369ba-5c73aaf]
at java.base/java.lang.Thread.run(Thread.java:1447) [?:?]
Caused by: java.lang.NoClassDefFoundError: net/milkbowl/vault/economy/Economy
at java.base/java.lang.Class.forName0(Native Method) ~[?:?]
at java.base/java.lang.Class.forName(Class.java:543) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:68) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:145) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
... 8 more
Caused by: java.lang.ClassNotFoundException: net.milkbowl.vault.economy.Economy
at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:160) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:112) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:490) ~[?:?]
at java.base/java.lang.Class.forName0(Native Method) ~[?:?]
at java.base/java.lang.Class.forName(Class.java:543) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:68) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:145) ~[spigot-api-1.21.10-R0.1-SNAPSHOT.jar:?]
... 8 more

Excellent SG Plugin, the best one with updates and the DEV is really nice, listens to the community and is going above and beyond for each customer.

Plugin: 100/10
Dev: 10000000/10

Awesome plugin! Will you be adding more features to this free lite version? I'd like to have 1.8.8 support