Celtx 3.3 is a 32-bit application . Apple officially dropped support for 32-bit apps in macOS Catalina (10.15) and later.

If you’ve tried everything and your Mac is too new for 32-bit apps, here are the best substitutes:

This guide assumes you are running macOS Mojave or High Sierra.

And if you cannot get it to run? Embrace or WriterSolo —the new generation of free screenwriting software that carries the torch that Celtx 3.3 first lit.