Why can't I play replays or galleries? Even if I finished them all? Thanks for response!
Replays are based on scenes you have seen. They should unlock as long as you go that route. Some earlier scenes are missable, the ones with colored (yellow) text can sometimes be missed. Galleries are similar, except they are based on the images themselves and all images of the scene must be seen.
You should see most of the scenes and images if you often save and return to yellow options, all if you visit every option you can.
The rarest case, some characters have scenes that show up in another character's storyline. If you are very curious about a specific scene you can ask here or on my discord. People are very helpful with hints and stuff!