I just tried to setup the MDK for 1.21.11 because my mod which runs in 1.21.10 doesn't run in 1.21.11 because it couldn't find the ResourceLocation class. (Minecraft crashed with the error that it couldn't find that class)

But after executing the command gradlew init eclipse genEclipseRuns as usual in my commandline, it fails after 1 or 2 seconds with the following error:

FAILURE: Build failed with an exception.

* Where:

Build file 'H:\Laufwerk(E)\Laufwerk(G)\Projekte\Duck Money (Forge 1.21.11)\build.gradle' line: 5

* What went wrong:

Plugin [id: 'net.minecraftforge.gradle', version: '[6.0.46,6.2)'] was not found in any of the following sources:

- Gradle Core Plugins (plugin is not in 'org.gradle' namespace)

- Included Builds (No included builds contain this plugin)

- Plugin Repositories (could not resolve plugin artifact 'net.minecraftforge.gradle:net.minecraftforge.gradle.gradle.plugin:[6.0.46,6.2)')

Searched in the following repositories:

Gradle Central Plugin Repository

* Try:

> Run with --stacktrace option to get the stack trace.

> Run with --info or --debug option to get more log output.

> Run with --scan to get full insights.

> Get more help at https://help.gradle.org.

BUILD FAILED in 4s

Does somebody know what I may miss here?

You need to add the Forge Maven as a plugin management repository in settings.gradle

pluginManagement {
    repositories {
        gradlePluginPortal()
        maven {
            name = 'MinecraftForge'
            url = 'https://maven.minecraftforge.net/'
        }
    }
}

If are not using the Mod Developer Kit (abbr. MDK) from our Files site, you definitely should. It sounds like you are using a different template, and since we don't make external templates, we can't ensure they will work out of the box.