Here's what I added and what is currently written in my pom.xml file that I guess is supposedly related to the NBTAPI shading:
This in :

<plugin>
       <groupId>org.apache.maven.plugins</groupId>
       <artifactId>maven-shade-plugin</artifactId>
       <version>3.6.0</version>
       <executions>
         <execution>
           <id>shade</id>
           <phase>package</phase>
           <goals>
             <goal>shade</goal>
           </goals>
         </execution>
       </executions>
       <configuration>
         <relocations>
           <relocation>
             <pattern>de.tr7zw.changeme.nbtapi</pattern>
             <shadedPattern>me.negai.wanddesigner.nbtapi</shadedPattern>
           </relocation>
         </relocations>
       </configuration>
  </plugin>

And this in dependencies:

<dependency>
      <groupId>de.tr7zw</groupId>
      <artifactId>item-nbt-api</artifactId>
      <version>2.13.1</version>
    </dependency>

And this on onEnable method in the plugin's main class:

        @Override
	public void onEnable() {
		if (!NBT.preloadApi()) {
	            getLogger().warning("NBT-API wasn't initialized properly, disabling the plugin");
	            getPluginLoader().disablePlugin(this);
	            return;
	        }
         }

Here is what it showed in the logs

Correct. That means it didn't work/run. Check the output when running the mvn clean install command.

I succeeded in fixing this, thank you so much for helping!

Look inside the jar, also do you actually build with maven and not the ide?

Look inside the jar, also do you actually build with maven and not the ide?

What do you mean by looking inside the jar?
And no I don't build with the IDE I build it with maven can it be the problem that causes this?

Look inside the jar if it is actually relocated or not. Like the actual file paths.

Look inside the jar if it is actually relocated or not. Like the actual file paths.

Oh I think now I see, there are 2 packages here, me.negai.wanddesigner (my package) and de.tr7zw.changeme.nbtapi... I guess it means it wasn't relocated right?

Correct. That means it didn't work/run. Check the output when running the mvn clean install command.