Thank you for reaching out with the issue you're encountering when trying to open Illustrator beta. Dealing with errors like this can be frustrating, but I'm here to help you find a solution.
The error message "The Operation cannot be completed because of an unknown error. [!FND]" indicates that there is an underlying problem preventing Illustrator beta from launching properly. Multiple users have reported this issue.
I hope Marina's or Junior Rocha's suggestions help you get Illustrator beta up and running smoothly again. If you are still encountering the error, please share your OS, AI beta build version and system details so we can further investigate the issue.
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