I think I solved it. I created a few folders to complete the path not found. I mean:
Inside that folder, I created a symlink to the folder containing drush:
mklink /d drush C:\Bitnami\drupal-7.53-0\apps\drupal\drush
Now the error message changed:
We could not find an applicable site for that command. [error]
... until I realized I had to execute the command (drush uli) in the htdocs folder. Once I launched it I got this message:
default does not appear to be a resolvable hostname or IP, not [warning]
starting browser. You may need to use the --uri option in your
command or site alias to indicate the correct URL of this site.
So I used the --uri option, et voilà...!
C:\Bitnami\drupal-7.53-0\apps\drupal\htdocs>drush uli --uri="http://localhost:81/drupal"
I hope this helps somebody