The error message implies that you are creating a task named ‘build’, which is no longer allowed if the ‘build’ task is already being supplied by the standard Gradle plugins.
Does the output point you at the offending code? For example, this:
apply plugin: 'base'
task build
tells me that the offending code is on line 2:
$ ./gradlew tasks
FAILURE: Build failed with an exception.
* Where:
Build file '/home/cdore/gradle-tests/gradle-dupbuild-test/build.gradle' line: 2
* What went wrong:
A problem occurred evaluating root project 'gradle-confforce-test'.
> Declaring custom 'build' task when using the standard Gradle lifecycle plugins is not allowed.
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED
What is the offending code in your case?