In order to be able to copy table cells the 'secure context' needs to be met. This means you will need to access the app over https://
More info about secure contexts: https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts
Also, the navigator clipboard feature needs to be available, but that should be available in all recent browser versions. (https://developer.mozilla.org/en-US/docs/Web/API/Navigator/clipboard#browser_compatibility)
Hope this helps!
Best regards, Ward