Photoshop

When I take an image and remove the background, I am unable to save as a PNG because that choice is not available. Why is this so? The choices I have are very limited and are not the what I need. I don't need it saved as a Photoshop doc, nor as a .tiff doc or a .jpeg doc. For certain uses, I need to choose "save as" a png doc, so why is this choice not available?

Thanks