First attempt to process a Mosaic of M31

I migrated from using the scripts on 0.99.10 to Sirilic and Siril 1.0.0 RC1 . Wow, it works great. I was trying a Mosaic of M31 so I used sessions for each panel of my Mosaic and stopped the process before registring the 4 sessions because I was not sure of the result when registring a Mosaic. I have to try it to see if the resulting image will have all 4 panels.
I had some errors in running the script that I was not able to identify so I will attach the log to the post in case there is a know answer.

M31 Log file.txt (2.2 MB)

Siril has no support for mosaic assembly. You may stack you points of view independently and assemble them in a specialized software I guess.
I don’t know why you have these error messages.
Note that your master dark and dark-flat must be calibrated by removing the master bias if you do this processing pipeline.

Thanks Vincent. Even if Siril has no formal way to process Mosaics, using “Sessions” and not stacking the sessions after registering gave me 4 .TIF files for each filter that I stitched with ICE (Image Composite Editor). It worked OK.
Yes, the Master Dark and The dark flats were calibrated with the Master Bias.
Results were so-so because one of my mosaic panel did not come out good (clouds I presume). Still don’t know the reason for errors in the log.