Intelli-J Error: "No Groovy SDK Defined"

Right-click on the root of your Grails project and select "Add Framework Support".
Select "Groovy".
The dropdown menu next to "Use library" will allow you to choose a Grails library.
If the "Use library" dropdown menu is empty, click "Create" and choose your Grails library directory. If you have installed Grails through SDKMAN, it will be located at: /Users/{username}/.sdkman/candidates/grails/current/ where {username} is your username.
To remove an old version of Grails: Select File -> Project Structure -> Global Libraries. Delete the missing Grails version.