One possible solution to this issue is to try installing the plugin from within Roblox Studio itself. According to a post on the Roblox Developer Forum, some users have reported success with this method1. To do this, open the Toolbox in Roblox Studio, go to your Inventory, and click the Install button for the plugin you want to install1.
If this doesn’t work, you may want to try some of the other troubleshooting steps suggested by Roblox Support for Mac users. These include restarting your computer, resetting your logs, and checking that your Mac OS version is compatible with Roblox Studio2.
If none of these solutions work, it might be a good idea to contact Roblox Support for further assistance. When contacting them, be sure to provide details such as your Roblox username, any error messages you’ve received, the model name of your Mac, and your Mac OS version2.
I hope this information helps and that you’re able to resolve the issue soon! 