Can anyone explain why she can't tell the female knight to me? I'm confused
Brigitte is directly related to Geibe Illgner, whose province is part of Mynes paper making business. However they only have limited knowledge as Myne is gatekeeping it to trickle through the right channels at the right time and making sure they pay their dues.
If Brigitte learned about some of the stuff in the workshop, such as the high quality feypaper, then Brigitte would have to choose between betraying Myne to further her family's fortunes, or betray her family and keep Mynes secret.
Damuel, comparatively, is a nobody. And he already knows Mynes big secret about being a commoner. If he wanted to start spreading secrets, he'd just be executed on the spot.
Damuel can't follow her at the end of the chapter because she's going to make Trombe paper using the Class A "Everyone who knows, dies" secret that Taue fruits absorb mana and turn into Trombe seeds.