A recent Mac update caused the older Eclipse version we use to not work on some Macs.
You will just need to replace the eclipse included with the installer. Open the Mod Creation folder and find the Eclipse program there. Move that to the Trash.
Next go to https://www.eclipse.org/downloads/packages/
Download the version of the Eclipse for Java Developers for your OS:
Open the .DMG it gives you.
Copy the Eclipse inside and move it to the Mod Creation folder you removed the old eclipse from. It needs to go back into the exact spot so that the Project Manager will attempt to use the new Eclipse.
Go ahead and open Eclipse and ensure it works. You will probably need to approve it since it is from the web.
If you get an error after this step, you will need to install JDK 11.
Head to https://adoptopenjdk.net/ and download OpenJDK 11
Finish installing that and Eclipse should work.
If you need help, or just don't want to do these steps yourself, we can go ahead and do a screen share. Just reach out to support.