How can I unlock the extra event？Should I use code or sth？
and I can’t view the extra scene in part 5， It means I need to replay that part or sth？
For extra event, this is only available to Level 3+ patrons on Patreon.
For part5 extra scene in gallery, I tested it with no problems, if your progress on Chapter1 is more than 5, it will auto unlock in the gallery.
Please make sure you don't use any other CG unlocking mods or anything else, most of the possible causes of problems that I know of so far are these things.