Where the Add Window function does not become available for a selected area, this may be due to adjacent glazing being too close to the solid wall. Glazing that abuts a solid wall where windows are to be added, may prevent placement of windows there as the joint condition cannot be verified. This should not be a common issue in the latest versions.
If this does happen, this may be overcome by moving the glazing away from the solid wall (eg. 3 feet / 1 metre). This may also occur where internal glazed partitions abut a solid exterior wall – the Add Window function may be disabled. You may temporarily move the internal glazed partitions back away from the exterior wall in order to place windows using FenestraPro and restore them when windows are placed.
The FenestraPro window family will also need to load into the Revit model and become active for use if you have not used this function before for this project. This may take a few minutes, please wait until this is complete. Note: The Project Browser in Revit should also be open when loading this FenestraPro window family to verify that the window families list has updated.
There is more information here in the Add Window Section.