Tandem_x product-multilooking in SNAP toolbox

SNAP requires metadata of your image if you want to use most of the SAR tools.
Open the TDX data by File > Import > SAR > TanDEMR-X and select the corresponding XML file instead to open the image in SNAP. This will ensure that all available information (date, product, product level, incidence angle…) is available to SNAP.

Please also see lvecis answer on this: