You don’t want to convert to bitmap for this to work. Both components need to be vectors for boolean operations to work.
create rectangle
create text
3a. if both objects are on the same layer the text should already be inverted from the rectangle
3b. alternatively, you should now be able to apply a boolean operation to remove text from rectangle
if you really want this to be a bitmap/raster you can now convert to bitmap. You will need to group the objects if you went with the first option.