Error: "'APPNAME' is not optimized for your Mac" when opening an application

Symptoms

You receive the following error in macOS Mojave when opening an application, where APPNAME is the name of the app:

"APPNAME" is not optimized for your Mac and needs to be updated.

This app will not work with the future versions of macOS and needs to be updated to improve compatibility. Contact the developer for more information.

example screenshot

You receive the following error in macOS Catalina or later when opening an application, where APPNAME is the name of the app:

"APPNAME" needs to be updated.

The developer of this app needs to update it to work with this version of macOS. Contact the developer for more information.

example screenshot

Cause

This error occurs because the application that you are opening is a 32-bit apps. Apple ended support for 32-bit apps and urges developers to update to 64-bit apps.

Resolution

If you are on macOS Mojave (10.14.x) or older, you can click OK to open the application.

If you are on macOS Catalina (10.15) or newer, the application will no longer open.