Unable to install Signal from App Store on iOS
Either the app developer is pushing out a price/code change, the App Store servers are down, or there's a problem with the app within the App Store.

When developers make a change to the app, including price, it can takes several hours for the change to propagate through the Apple servers.  You can keep trying for a few hours and see if this clears itself.

If after 24 hours you still see the same error, you can contact the developer's support (usually from their website) and let them know.  It's not uncommon for this happen when the developer hasn't made any changes at all; there's dozens of comments about this throughout the years in the Apple developer forum.

The developer is the one losing revenue so it would be in their best interest to contact Apple about the problem and work with them to resolve it.