Hi @Sara.Aparicio,
I suggest you go inside SNAP/ Tools/Manage External Tools the select sen2cor and click edit.
In the screen that would appear 2 items need your attention 1) Configuration Parameters and 2) System Variables.
I have attached sceenshots for what should be there (adapt to your computer). Please note that for $PYTHON_BIN you have to navigate (click …) to the python2.7 under your Anaconda2 directory.
If this doesn’t work I would advice to remove sen2cor and re-install it again.
