Had a similar error trying to use Fabric's Twitter Kit
Error:Execution failed for task
> Crashlytics Developer Tools error.
ERROR - Crashlytics Developer Tools error.
java.lang.IllegalArgumentException: Crashlytics found an invalid API key:
Check the Crashlytics plugin to make sure that the application has been
Contact email@example.com for assistance.
After login in Fabric, download
the AndroidStudio plugin and let it configure everything all worked
(Btw, I really don't like this setup flow)
EDIT: It also can be done without install the AndroidStudio plugin.
Follow these instructions from the Fabric site https://fabric.io/downloads/gradle