Question related to snap platform

Hi, I am new to SNAP was ok. downloading the T-o-A data, view metadat, view world map, view single band s, multiple bands, cropping, spectral analysis, but when tried to convert data to B-o-A DATA I faced the error
" working path does not exist"
Also, when using BAND MATHS to create NDVI/NDWI faced the error
“Java.Lang.Null pointer Exception”
I tried hard to solve the problem via the step forum questions/answers but most of the answers are related to command line processing and i am using menu driven commands in snap sentinel 2 application platform.
Could you please help me
Thanks alot

Have you checked out the tutorials?

can you explain this in more detail?