Open the sprite in Aseprite and try converting a layer to a background layer. You can find the conversion under Layer in the top menu bar or by right clicking on a layer’s name in the timeline to open a context menu. When a layer is a background layer, it’s name will be underlined in the timeline. The layer’s name will be changed to “Background.” This is assuming the bmp is in RGB color mode.
I found this info by skimming a past Github issue.
If adding a background doesn’t work, it might help to say what version of Aseprite you’re using (1.2.40, 1.3, whatever).