Hi 育立陳,
I have checked that you have a free account for Adobe Sign.
In Adobe Sign free account, you get only 5 transactions in the 30 days rolling window.
Could you please confirm if you get the error only when you send a document from the mobile application or on the desktop as well?
We will be waiting for your response.
Let us know if you need any help.
Regards,
Meenakshi