To open multiple instances of Spine on macOS, you can use the Terminal command open -n -a /Applications/Spine/Spine.app. This command should allow you to run multiple instances of the application. If you are encountering an error when trying to do this, it might be helpful to check the error message for more details. Sometimes, issues can arise from corrupted files or conflicts with other software, such as virus scanners. Ensure that your Spine installation is not corrupted and that no other software is interfering with its operation. If the problem persists, you might want to try reinstalling Spine or checking for any updates that might address the issue.

If you click Report... does it display any additional information?
Checking the log when this happens could also be useful in understanding the cause:
https://esotericsoftware.com/spine-troubleshooting#Spine-Troubleshooting

AssafHayut What version of the editor are you trying to launch multiple? I see from the screenshot you attached that you are using the latest Spine launcher (4.2.18), but I couldn't tell which editor version you are using. Once we know that, we will see if we can reproduce the problem.

AssafHayut If you did not specify a launch version in your command, the last launched version should be launched. You can check the last launched version in the version field of the launcher screen:

For your information, when starting Spine via the command line, you can use the -u or --update parameter to specify the version to start. This is explained in the following section of the user guide:
https://esotericsoftware.com/spine-command-line-interface#Editor