How to troubleshoot this in Global Configuration Management: "Error occurred when getting project areas. Project area selection is unavailable."

A good way to troubleshoot this is:
1. Hit the F12 key in Firefox/Firebug or Chrome, to bring up the Network tab
2. Reproduce the error
3. Check the Network tab for the result of this URL:
https://SERVER/qm/service/com.ibm.rqm.process.deployment.rest.IPermissionRestService/initialProjectAreas?maxResults=50&pleEnabledOnly=true
 (Note: this is for QM; the URL may look different.)

4. Run the same URL in a browser.
5. Also try removing the pleEnabledOnly=true parameter:
https://SERVER/qm/service/com.ibm.rqm.process.deployment.rest.IPermissionRestService/initialProjectAreas?maxResults=50

In our case we found that the pleEnabledOnly=true parameter caused no project areas to show from RQM. This triggered the misleading error message:
"Error occurred when getting project areas. Project area selection is unavailable."

This misleadning error message is a known defect:
https://jazz.net/jazz02/web/projects/Rational%20Quality%20Manager#action=com.ibm.team.workitem.viewWorkItem&id=152258

The solution is to enable global configuration for your project areas in RQM and you can proceed to reference them from GC.