I really like IDEA for developing.
I worked on a golang project and I organised my code into separate packages
The project built without problem in console (go build).
But, in IDEA I got this error:
$GOPATH/go.mod exists but should not
The solution is,
after the module support is turned on (IDEA/preferences/languages/go/Go modules : checkbox), it cannot exist with GOPATH, so just remove the project from GOPATH at IDEA/preferences/languages/go/GOPATH