How to save raster created by band math

There is a problem with exporting the raster created by bandmath .
Could someone explain me here how to export it in geotiff

File > Export > GeoTIFF > subset > select your wanted band

refer: ESA SNAP Export

Does the icon for the bandmath generated band have a
image for “virtual band”? If so, uncheck the
virtual box selection in the Band Maths Window. A virtual band does store the actual raster values, only
the formula used to generate the values. For me, export+subset converts the band to a regular band (the “V” icon changes to a regular icon), but no output file is written.