The “Copy message” button in the Signal App on iPhone doesn’t show up when you long press the text (on the pop-up ribbon like other messaging apps).
The copy icon is placed in the bottom Tab Bar — an awkward UX error for such a mature app.
For developers: overriding the given operating system’s native gestures, for basic tasks like copying and pasting, is a really bad idea. There are plenty of other ways to add character to your apps, but just keep the look, feel, and gestures for regular tasks close to the native OS ways.
Install Signal here: https://signal.org/install/