Update: We just rolled out a fix for this issue. Thanks for your patience—you should be able to pin on the mobile web again!
Right now, we're having some issues with Pinning on mobile browsers and we're doing out best to figure out what's wrong. Hopefully you'll see this problem resolved soon!
In the meantime, I recommend downloading our app for iOS or Android from your App Store for a better experience on your mobile device. Thanks for your patience!