Tolerance is the lever
Raise it to eat a gray halo around the subject. Lower it if the cut starts eating the object. Start near the default and move once.
The file has to be PNG or WebP
JPG cannot hold transparency. The download is a PNG so the checkerboard stays empty where the white was.
What fails honestly
A subject that touches the frame edge, a shadow that is not the backdrop color, or a photo of a person. Use a real cutout tool for those.
After the cut
If the PNG is too large for the form, compress it or export WebP. A photo-sized PNG often misses a 200 KB cap.