How to add Freemarker to Plugin? (gradle)

Please post the full snippet of your build.gradle.kts, not just a single line

...
plugins {
    // Java support
    id("java")
    // Kotlin support
    id("org.jetbrains.kotlin.jvm") version "1.5.0"
    // gradle-intellij-plugin - read more: https://github.com/JetBrains/gradle-intellij-plugin
    id("org.jetbrains.intellij") version "0.6.3"
    // gradle-changelog-plugin - read more: https://github.com/JetBrains/gradle-changelog-plugin
    id("org.jetbrains.changelog") version "0.6.2"
    // detekt linter - read more: https://detekt.github.io/detekt/gradle.html
    id("io.gitlab.arturbosch.detekt") version "1.14.2"
    // ktlint linter - read more: https://github.com/JLLeitschuh/ktlint-gradle
    id("org.jlleitschuh.gradle.ktlint") version "9.4.1"

    id("org.freemarker.freemarker") version "2.3.14"
}
...

// Configure project's dependencies
repositories {
    mavenCentral()
    jcenter()
    maven {
        url = uri("https://plugins.gradle.org/m2/")
    }
}
dependencies {
    detektPlugins("io.gitlab.arturbosch.detekt:detekt-formatting:1.14.2")
    implementation(kotlin("stdlib-jdk8"))
}
...

You must use "dependencies" block, not "plugins" which defines plugins for Gradle build itself. https://docs.gradle.org/current/userguide/building_java_projects.html#sec:java_dependency_management_overview

Thanks...this works for me.

I use an local Dir with a jar and renamed the jar "freemarker.ar" into "2.3.31.jar"

// Configure project's dependencies
repositories {
    mavenCentral()
    jcenter()
    flatDir{
        dirs("<YOUR_DIR>/apache-freemarker-2.3.31-bin/")
    }
}
dependencies {
    detektPlugins("io.gitlab.arturbosch.detekt:detekt-formatting:1.14.2")
    implementation("org.freemarker.freemarker:2.3.31")
    implementation(kotlin("stdlib-jdk8"))
}