We really need more information to be able to provide any assistance.
What app are you using to attach? Where did you get the file? What are you trying to attach it to?
The only way I ccould send an emoji to anyone was to export it from the studio then attach it in a text message. Directly sharing gave me the same error "Unable to attach. File not supported".
Then I got to thinking. If I can attach the same file without an error then maybe AR Emoji doesnt have permission? Turned out it did not have permission to use the phone so I granted it -
Settings > Apps > AR Emoji > Permissions > Phone = Enable
Went in the emoji studio, share, messages, contact... It worked. A 1.1 mb emoji sticker was inserted and no error... Opened a new text message, tapped the sticker icon on the keyboard and inserted a different emoji I created earlier. So thats working as well.
Im hoping this could solve the error message for other people also.