Hello Archix,
Rotate Object option is available for unreferenced models only, so if you have already used coordinates of the camera or marker locations, this option will be unavalable.
Probably, you are using geotagged images and PhotoScan automatically loads the GPS data from the image EXIF. If so, then you need to clear the coordinate data in the Reference pane and switch the referencing system from WGS84 to Local Coordinates.