This error is due to the fact that the new lossless compression format introduced by Sony and Nikon in their latest camera models is not yet supported by Nebulb.
The other formats (uncompressed or lossy compression) can be imported directly.
In the future this format will also be supported by Nebulb, but not at the moment.
That said, as in this case you have already captured the photos, I recommend that you convert them first to DNG to be able to import them into Nebulb without losses.
And for the next photos you are going to process with Nebulb you can set them in a different RAW mode than lossless compressed (to avoid having to do a previous conversion).