Hi Katerina,
Yes, you are right it is the missing info in the metadata file that prevents you from co-registration.
I was having the same issue, but like I said that was due to extracting [S] matrix in the same folder.
Although I have never used L1.5 data, I guess extracting in SNAP is same for all data types.
Have you extracted the data using any other software before importing in SNAP? If so, then that maybe the error.
You would always want to use SNAP first for all extraction.
HTH.
-Rosh