The “Copy message” button in the Signal App on iPhone doesn’t show up on the pop-up ribbon (like other messaging apps), but rather in the bottom Tab Bar. An awkward UX miss for such a mature app!
Do not override the native OS’s gestures for basic tasks like copying and pasting. There are plenty of other ways to add ‘character’ to your apps, just keep the look, feel, and gestures for such regular tasks close to the native OS ways.
Also, Install Signal: https://signal.org/install/