Is it possible to subset the image with the polygon shape (more than four edges) instead of polygon bounds using subset-operator/snappy?

[marpet]

Yes, this is possible. You can specify a polygon in WKT-format.
For example with gpt on the command line:

The ‘geoRegion’ parameter can also be used when using snappy.
But in the end always the bounding box of this region is used. Because in the end a raster needs to be created and rasters are rectangular.

There is the Land/Sea Mask operator which can mask the area outside of the polygon after the subsetting.
See:

SNAP: Clip Raster by Shapefile

The Land/Sea Mask operator should actually do what you intend. The error message seems a bit strange. Have you set all parameters?

Subset using Vector file

Just for the records, the option to use a shapefiles to subset/mak a products is also well-hidden within the “Land/See Mask” operator (under Raster --> Mask).

Sentinel-2 problem with Land/Sea mask tool

Problem solved! It is necessary to do Reprojection before using Land/Sea Mask. Strange, because even before that shp layer is showed in suitable place in the view. However, with processing path as follows, everything is correct: Resampling (demand by SNAP even if you finally need only bands with the same spatial resolution, f.e. 2,3,4) Reprojection Import vector layer Land/Sea Mask. Finally, it was connected with CRS, so thank you for inspiration and motivation!