There are multiple Python packages called “snappy”, so it is quite likely that your Python39 is using something other than ESA SNAP snappy.
The above document describes ways to make ESA SNAP snappy available, including use of
sys.path.append("<ESA_SNAP_snappy_dir>"), in your Python scripts. This has the advantage
that it makes it clear which snappy is being used, but you need sys.path.insert(0, <ESA_SNAP_snappy_dir>) to ensure that ESA SNAP snappy is found before some other snappy.