Thanks for the quick response. I think I've figured it out: I was creating 1080x1080 art. Previously I used the Instagram Story template, which I then had to reformat, but then decided to use the custom size. I then used the rectangular tool to create the canvas, thinking it was the artboard. At that time I didn't even know that there is an artboard. I think it's the same as a canvas. After I've created new pieces, creating an artbard, everything worked again. I also was able to add the missing artbord.