Hello Stefano,

It seems like the issue is with the coregistration process in your longitudinal study. The error message indicates that the mean functional file is missing, which is essential for coregistration. I suggest you first ensure that the realignment process has been completed correctly, or you could try using the "first functional volume as reference" option as the error suggests. This might resolve the issue by bypassing the need for the mean functional file. Good luck with your study!