Are you using the alpha version of GradleRIO or using Eclipse? You need to upgrade to the 2019 suite of tools. See http://wpilib.screenstepslive.com/s/currentCS/m/java/c/88899 for installation instructions. For Eclipse, there’s a project upgrade tool; if you’re using the alpha version of GradleRIO, you will need to create a new project and copy your code in manually.
I updated to the 2019 Suite of tools recently, but made my current project over the summer to try and “get some work done”. I’ll copy my code now. Thank you.