Android Studio 2.1: error: package org.junit does not exist

add this dependency to solve your issue

testCompile 'junit:junit:4.12'
compile 'junit:junit:4.12'

I changed TestCompile to androidTestCompile and it's worked without problems.

testCompile 'junit:junit:4.12'

to

androidTestCompile 'junit:junit:4.12'