There are somes issues that could be related with your problem:
- Path too long: this was solved in a previous version of SNAP, but the fix requires that the short path is enabled in Windows. (Can not open S2 xml datafile). If this is the cause of your problem, if you copy your product to a shorter path, it should work.
- Some users have had problems when unzipping the product, they fix them by using 7z (Can't open S2A L1C data xml)
Please, if the above solutions do not work, could you tell me what is the product to download it and test?