Do you use the latest version of Eclipse (NEON) ? If my guess is right, System Workbench for STM32 is not compatible yet due to dependencies missing with features removed from Eclipse NEON. The issue has been reported and an update will be published as soon as possible.
If you don’t want to wait the next update, the only solution is to download the older version of Eclipse (MARS) then install the System Workbench plugins.
Sorry for inconvenience.