Tandem_x product-multilooking in SNAP toolbox

Dear all,
I am working with Tandem-X data. I have intensity image in tif format and I tried to do multi-looking but I got this error:
‘Input should be a SAR product’
Would you please guide me?

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:

Thanks for your answer. I think the problem is that I only have tif image because my supervisor sent this one to me and I should have the whole package, not only tif format.