Screenshot and let me see if I can figure that out.
Discussion
Well here's what your reply looks like. Clearly the zap button should be right next to the like, right? 🤔 
I don't know what app you are using, but some of them does not have the feature that others have. Maybe if that's the case then you can try other apps. I use Plebstr, Amethyst, and Iris. I think there's Damus for iPhones.
Update: damus only allows profile tip from now on. No more zaps on contents.