Unresolved reference error after upgrading gradle

I upgraded Gradle from 4.0 to 5.0. I made this change in the config:

build-tools = {
             1.0 = {
                 ...
                 Gradle = 5.x;
             };

After the change I am getting an unresolved reference error in a parent package:

> Task :compileDebugKotlin FAILED
e: /Users/.../MainApplication.kt: (18, 19): Unresolved reference: axle

Any ideas on how I can debug or resolve this issue?