error sign "digital signature invalid"

Hello eejiwon,

Welcome to Apple support Communities.

I understand that you are getting an error saying that the digital signature is invalid when you try to open the installer package.

To resolve troubleshoot this, follow the steps.

1) This issue might be caused due to the software being installed is very old and your Mac ( Apple ) is not able to verify the validity of the certificate .

2) It may be corrupted, then you need to download a new copy of that installer from the Internet or get a new copy from somewhere.

3) Your Mac might not be compatible to run that software ( check the requirements with the product vendor/company ) .

4) If it is the issue with apple verifying the certificate then these steps should help you.

Steps to bypass an app from being required to verify the certificate validity.

#1. Open System Preferences.

#2. Go to Security & Privacy and select the General tab.

#3 If you've been blocked from opening an app within the past hour, this page will give you the option to override this by clicking the temporary button 'Open Anyway'.

#4. You'll be asked one more time if you're sure, but clicking Open will run the app.

#5. This creates an exception for that app, so you'll also be able to open it in the future without having to repeat this process.

This way you can permit that application from being blocked again.

To Allow All Apps from anywhere follow these steps. ( This will make your Mac to run any app from anywhere even without having a valid certificate. Make sure to disable it, if your work is done as it poses a threat for other malicious apps or Software being installed. ) ( Note: You will have the install from "Anywhere" option by default if you are running an earlier version of macOS 10.12 Sierra as that option has been removed from macOS 10.12 Sierra and later. )

#1 Go to System Preferences > Security and Privacy > in the General tab > you see two options, Allow apps downloaded from a) App Store, b) App Store and identified developers.

#2 There is a third option called " Allow apps from anywhere " this used to be an option in earlier versions of macOS, but disappeared when macOS Sierra arrived. However you can get the Anywhere option back.

#3 Open " Terminal " and enter the following code to get you anywhere option back.

sudo spctl --master-disable

#4 Now press Return, and you will be asked to enter your password. Once that's done, open System Preferences (if it's already open, you'll need to quit it and restart to see the new options) and go to the Security & Privacy section.

#5 A new, third option will have appeared allowing you to 'Allow apps downloaded from: Anywhere'. You'll have to click the padlock icon to make changes to the settings on this page.

If you want to remove the install from anywhere option for security reasons, enter this code in terminal.

sudo spctl --master-enable

This will hide the "Anywhere" option.

I hope you find this helpful and solves your issue . Please feel free to reply back if you need any more help and I will be happy to help you further.

Cheers!