https://outsiderartisan.neocities.org/succubus-stories/CHANGELOG.html#guide-priscilla-romance
Priscilla cannot be romanced after she moves to the bandit camp early in Chapter 3, so you need to do it before then. However, not romancing her will also open up different sets of options later. While due to the nature of the game, you obviously have pretty open relationships in the romances, only one major character can be romanced in the story, as it has some effects on the endings, so missing Priscilla isn’t too big a deal.