I'm unable to reproduce your errors. Doodle Studio works for me on Unity 2018.2.3 when you import it in a clean project. This could be caused by other libraries you have installed in your project.
Try making a new project and importing the doodle studio package. If you get errors, let me know what those errors are, the specs of your computer and your OS version.
If there's no errors, go back to your project and try deleting the doodle studio folder and importing it again.
If there's errors in your existing project but not in a empty one, the culprit is somewhere in your project and out of my reach.