The Preview version of a Workforce Analytics site (accessed via analyticsX.successfactors.com/Preview or analyticsX.sapsf.com/Preview) does not allow Workforce Planning testing.
SuccessFactors Workforce Analytics with Workforce Planning enabled on Production
Unfortunately Workforce Planning is not available on the Preview site, for any instance.
If you wish to test WFP before releasing it to all users, you will need to use WFA roles on Production to restrict it to the testers at first. Workforce Planning and Workforce Planning Admin are "Actions" that can be individually restricted.
KBA , LOD-SF-WFA-WP , Workforce Planning , Problem