Hello! Let me explain about making transparent background. It's not possible with Adobe Sketch app for Android, but it's available on Creative Cloud.
Save the picture to the gallery and click Upload in Creative Cloud's My Work to import the image. You can also select an image to remove the background by clicking Bae Gye-geo. If you select and proceed, the background disappears.
If you proceed like this, you can remove the background of all images. It may be a bit of a hassle, but if you do this, you'll be able to manage the picture files in Creative Cloud. 🙏🏼