How to remote trigger parameterized jenkins jobs with R code?
You aren't specifying a user ID in your request, so it is going to be using the anonymous user. Your anonymous user has to have read access to the job (or job->read in global perms) you are trying to trigger, or it won't be authorized to access the job/PROJECT uri to trigger the build.
So you have to either specify a user ID with the proper read access in the url, give the anonymous user read access, use project-based matrix auth, or you can use the Build Token Root plugin to provide a different URL that doesn't require read access to the job. Then you only need the token.