Thanks for coming back to the Community! That error message generally is caused by either using an unsupported Emoji, or by using our Legacy Newsletter templates. Following the above post about using Emojipedia.com should ensure you're using a valid emoji.
If on your campaigns page you see that you're using the Legacy Newsletter, just create a new campaign in our new Third Generation Editor, and that should resolve the issue for you.
Aaron H Community & Social Media Support
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