Unfortunately, it seems you cannot draw on top of a photo, only on a blank canvas. Not sure why it is restricted this way unless I am missing something.
Thank you for reaching us. In the drawing feature, we have two options: draw from scratch, which is on an empty canvas, and the other one is you can select the templates available and draw on them. If you are looking for something else you can always share your suggestion through https://adobe.ly/3RXwtbB to let our developtment team know and considerit for future updates.