When I compile the template without any modifications using a 1.15.2 buildtools spigot.jar to satisfy the local dependencies, I get this error when running on tuinity (a fork of papermc that adds unticked view distance, i have not tried spigot). The getId() method of materials is deprecated, but I see you using it in the problem code. I think you have to use the name() method and reference the materials by their names (String) instead of by ID (int).
How are you able to make this work on 1.15.2? The only way I can make it work is to completely rip out inventory manager, which sucks, because I want to use that feature in the plugin I'm writing.
If you are going to advertise support for 1.15.2, then this really needs to be fixed.
[03:22:32 INFO]: [TemplatePlugin] Enabling TemplatePlugin v0.0.2.2
[03:22:32 INFO]: [TemplatePlugin v0.0.2.2] === ENABLE START ===
[03:22:32 INFO]: [TemplatePlugin v0.0.2.2] Plugin Version V0.0.2.2
[03:22:32 ERROR]: Error occurred while enabling TemplatePlugin v0.0.2.2 (Is it up to date?)
java.lang.ExceptionInInitializerError: null
at fr.maxlego08.template.Template.onEnable(Template.java:25) ~[?:?]
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:263) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:364) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:483) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.enablePlugin(CraftServer.java:470) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.enablePlugins(CraftServer.java:384) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:488) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:299) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:895) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.lang.IllegalArgumentException: Cannot get ID of Modern Material
at org.apache.commons.lang.Validate.isTrue(Validate.java:136) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at org.bukkit.Material.getId(Material.java:3334) ~[patched_1.15.2.jar:git-Tuinity-"e0cce97"]
at fr.maxlego08.template.zcore.utils.ZUtils.<clinit>(ZUtils.java:244) ~[?:?]
... 10 more
[03:22:32 INFO]: [TemplatePlugin] Disabling TemplatePlugin v0.0.2.2
[03:22:32 INFO]: [TemplatePlugin v0.0.2.2] === DISABLE START ===
[03:22:32 INFO]: [TemplatePlugin v0.0.2.2] === DISABLE DONE (0ms) ===