what data are you using?
In the case you downloaded Sentinel-1 (VV and VH): Polarisations cannot be generated, they are a product of sensor configuration. If no horizontal waves were emmitted by the satellite at the time of image aquisition, it is impossible to get HH information from it.
Sentinel-1 (and many other SAR satellites) only operates in single or dual polarisation. For the polarimetric matrix generation you need quad-polarized data (HH/HV/VH/VV) which is acquired by only few SAR satellites (ALOS, Radarsat, TerraSAR-X).
ALOS PALSAR data can be downloaded for free. But they are only available as archived data between 2007-2011 and 2002-2012 respectively.
