It appears that you are using the old (and only) PBI form of EasyPBI, as this is a known bug that only appears when using it for the first time.
There are two possible solutions:
1) Upgrade from PC-BSD 9.0 to the new 9.1-RC2 testing release (since the new version of EasyPBI is now included with PC-BSD 9.1) and uninstall the PBI for EasyPBI. This will also allow you to use the new features for EasyPBI like the module editor and additional improvements when creating/building modules.
2) Startup EasyPBI from the command line (just type "EasyPBI") and start the build process again (it will fail again). Look at the command line, and it will display the command that it is trying to run to create the PBI. Simply run that command manually from the terminal *ONCE*, and after that first PBI gets going it will properly fetch the build environment and save it on your system. Once that is done, you will be able to then use the EasyPBI GUI to build PBI's without any problems after that (I would wait for the build you started to finish though, just because trying to build two PBI's at once it quite a burden for modest desktops/laptops).
Let me know if you run into any difficulty with it!
~ Ken Moore ~