Newb here at ACF, attempting to be of assistance because as a Tipper I have run into this situation before on CB: in my situation, I had PLENTY of tokens and kept trying to Tip the model, but CB wouldn't let me for some reason or another. Turns out, simply refreshing the page (or maybe it was logging out and logging back in again?) fixed it.
If this is NOT your situation, feel free to completely ignore my suggestion. I'm SURE others above my pay-grade will chime in relatively soon