For my first install of Sweethome3D, I installed the snap version V6.6 through the Ubuntu Application installer. After the first series of problems I had, I removed that installation then installed the V7.2 from http://www.sweethome3d.com/download.jsp following the instructions you posted some time ago. It was on that install that I installed the Plugin Manager and discovered that several of the plugins (including roof generator) are not compatible with V7.2.

In an attempt to get a version that the Roof Generator will work on, I removed the V7.2 installation and reinstall the Snap version V6.6. I am still getting the error message: