Fist thing, you should remove this one version 5, please take a look at the following posts when you uninstall the version 5,
and install the latest version 6. from here, after successful installation, from help tab, check for update
Source : SNAP Download – STEP
This version should be compatible with latest version of SNAP, However, it has problem under investigation,
Take a look at this post,
How to install and implement sen2cor version 2.08 please take a look the following thread,
As I mentioned sen2cor has problem of implementing directly from SNAP, Nevertheless, you could implement it from the command shell,
Take a look at the following post,