[Solved] Build fails: "Plugin with id 'forge' not found."

SOLUTION: Run as administrator.

Whenever I try to build my mod, it fails:

C:\oresplus>gradlew build

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\oresplus\build.gradle' line
: 18

* What went wrong:
A problem occurred evaluating root project 'oresplus'.
> Plugin with id 'forge' not found.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug
option to get more log output.

BUILD FAILED

Total time: 5.547 secs

Here's my build.gradle file:

buildscript {
    repositories {
        mavenCentral()
        maven {
            name = "forge"
            url = "http://files.minecraftforge.net/maven"
        }
        maven {
            name = "sonatype"
            url = "https://oss.sonatype.org/content/repositories/snapshots/"
        }
    }
    dependencies {
        classpath 'net.minecraftforge.gradle:ForgeGradle:1.2-SNAPSHOT'
    }
}

apply plugin: 'forge'

version = "1.0-alpha-2"
group = "com.outlook.siribby.oresplus"

minecraft {
    version = "1.7.10-10.13.0.1180"
    runDir = "eclipse"

    replaceIn "OresPlus.java"
    replace "@VERSION@", project.version
}

archivesBaseName = "oresplus-" + project.minecraft.version

processResources {
    inputs.property "version", project.version
    inputs.property "mcversion", project.minecraft.version

    from(sourceSets.main.resources.srcDirs) {
        include 'mcmod.info'
        expand 'version':project.version, 'mcversion':project.minecraft.version
    }

    from(sourceSets.main.resources.srcDirs) {
        exclude 'mcmod.info'
    }
}

I've tried building my other mods, and the same error occurs. Does anyone know why this happens?