I can't turn the black background into transparent

[soursop]

Using Gimp 2.10 on Mac.

How can I remove the black background in this image? I want it to be transparent.

https://dl.dropboxusercontent.com/s/t0ku...s.png?dl=0


In my Colors menu, the "Color to Alpha" option is not selectable.
I tried Bucket tool, Color Erase, but that doesn't seem to work. I'm such a rookie.

[Blighty]

Worked fine here. Black went to transparent without problems.

Best method is along these lines:
https://www.gimp-forum.net/Thread-superi...on-picture
but substitute black for white.

[soursop]

Thanks, Blighty. With some adjustment with threshold, and choosing the wing instead of the black background, it worked.