An app refuses to run

If an app’s icon shows a no-entry sign, it’s incompatible with your version of macOS. Check with the app’s maker for a new version. If an alert warns that an app is ‘not optimised’, it’s 32-bit and will need a 64-bit update, but for now (and in the upcoming macOS Mojave) you can still use it. If an alert says the app ‘can’t be opened’, see page 28.