Upgrading Gradle to 6.x

Question

I am updating the version of Gradle of my application, it was with version 2.3 and I am going to 6.7.1 and I am not able to solve this problem:

Could not set Unknown Property 'testClassesDir' for task ':systemtestRun' of type org.gradle.api.tasks.testing.Test.

build.Gradle:

task systemtestRun(type: Test) {
description 'run tests'
testClassesDir = sourceSets.systemtest.output.classesDirs
classpath = sourceSets.systemtest.runtimeClasspath + sourceSets.test.runtimeClasspath }

Someone could give me a hand?