I know it's been 4 months, but it may help someone else. You need to do 3 main changes,

1. In your settings.gradle, you should be using a recent version of the workspace plugin.

2. In your rest-config.yaml file, you should specify a compatibilityVersion. Add / edit a line in the file to read:

   compatibilityVersion: 6
   

3. Finally, in the -impl folder (where your rest-config.yaml file is), add a gradle.properties file and put the following line in it:

   com.liferay.portal.tools.rest.builder.version=1.0.338
   

This was shamelessly copied from https://liferay.dev/blogs/-/blogs/restbuilder-transformutil-errors