What to say when she says that's sweet but I'm not into dating apps

Most of the time, a line like this is about the setting. The match happened on a platform where everyone is swiping, the first message felt like a pitch, and she didn't want to be sold to before she'd decided anything. That's a different thing from being turned off by you. Read the rest of her message and her profile for clues. If she names something specific you said, or asks a follow-up question, the door is open and the app is just a backdrop she'd rather leave. If her whole response is that one sentence and it's flat, she's telling you she's done with the format and probably not much more.

The mistake people make is treating the app as the obstacle to solve. You don't need to convince her apps are fine or explain why you use them. Her preference is simply information about how she likes to meet people. Accept it as stated, and the next move becomes obvious: find out whether there's anything to talk about outside the app at all.

When she says she isn't into apps, the first thing she needs is to hear you take that on board without flinching. A quick, easy acknowledgement does that. Then give her a reason to answer something other than the app question. Ask about the thing you actually liked, the book or the trip or the thing on her profile, because that tells her you were paying attention to her, not to the swipe. Keep it light enough that replying costs her almost nothing. Don't ask her to explain her stance on apps, and don't ask whether she'd consider meeting in person in the same breath. One easy question about something she cares about does more work than any clever line.

The move to avoid is answering her with a question about her reasons. That asks her to defend a preference she's already stated, and most people will simply stop replying rather than do that.

If she replies warmly, keep the thread moving at her pace and don't rush to propose a date. If she doesn't reply, give it real space before deciding anything. A few days is normal, and a week of nothing is her answer. Don't send a second message that restates the first, and don't send a message that asks whether she saw your last one. One short follow-up is reasonable if the conversation was going well and you've left a natural opening, but it should be a new thing to talk about, not a nudge. If that also goes unanswered, let the thread go. Stop there. Trying a third time, or moving the conversation to another app to get around her preference, is the point where you stop being someone she'd want to hear from.