Restoring /system but not /data within TWRP I want to restore a TWRP backup on my Samsung Galaxy S6 (SM-G920F), to get back from a custom ROM (9.0) to Stock (7.0).

Restoring /system goes fine, but TWRP throws an error when it comes to restore /data : "extractTarFork() process ended with ERROR: 255
My question is : can I restore only /system and boot my original ROM ? If that doesn't work, how can I fix this error ? (I don't have a sdcard)
[Can] I restore only /system and boot my original ROM ?


It depends upon what lies in the backup. If you took the backup of your stock ROM and you're referring to the same backup, then yes, restoring /system using that particular backup would restore stock ROM. But you would need to restore /boot as well since that is part of stock ROM too.

However, if you must restore a different ROM, even if it is a stock ROM, it is a good practice to do a clean flash or atleast overwrite existing /data and /cache files with the ones in the backup to avoid conflict between various versions of files/apps which can cause bootloop or battery drain or whatnot. 

In your case, since you cannot restore /data from the backup, I advise you to take a complete backup of your current ROM (including userdata), then reset /data (except /data/media) and /cache, and then restore /system and /boot from stock ROM backup. In this way, you would end up with a pristine stock ROM and can always restore /data when you manage to fix that error you mentioned. This is the path of least problems.