Unable to use transparency fill bucket

Hi all, I am taking Amy Bradley's Skillshare course "Repeat Patterns in Adobe Fresco with Pixel and Live Brushes" At around the 3 minute mark of Lesson 4 she shows how to "cut out" parts of shapes using the transparency fill bucket and she is on a pixel layer. I tried several times to fill a square template floating over a green pixel square with transparency and I kept getting a message popping up in blue in the upper right hand corner of my screen saying "Cannot fill this type of layer." I was, however, able to fill the square with a solid color, red.  I did several google searches an only came up with the idea that transparency fill can only be used on vector layers. Is this true? Her video makes it appear otherwise. I decided to start fresh using a vector layer to make my green square and sure enough, sliding up the template and "cutting it out" with the transparent fill worked like hers did, so I was able to do it but only on a vector layer. I checked her actions leading up to it and it clearly shows her  selecting a pixel layer so I am at a loss. Do you know what I could be doing wrong? Thank you!