Uploading tests to device farm and getting "Could not find test jar" error

I am trying to upload my test framework to AWS(Amazon Web Services) Device farm and I am getting an error saying "There was a problem processing your file. Could not find test jar." I am using Gradle as the build tool.. I don't understand how to create the jar file. I have updated the jar file and zip.xml file. Instead of using mvn clean package –-DskipTests=true I am using gradle clean package –-DskipTests=true to create the jar and it gives an error saying "Task 'package' not found in root project". Can anybody help me in creating the jar file.