"The account does not exist in the Cloud API" - Whatsapp Business API problem with test number

In the page setup on Meta For Developers it says that the account (test number) does not exist in the Cloud API. I was previously able to run tests normally with the provided test number (receiving messages on my personal phone), but since yesterday afternoon, I've been getting some message like:

"Failed to send message. The account does not exist in the Cloud API. Use the /register API to create an account first."

Making the request in Postman, I have the following response:

"error": {     "message": "The account is not registered",     "type": "OAuthException",     "code": 133010,     "error_subcode": 2593006,     "is_transient": false,     "error_user_title": "The account does not exist",     "error_user_msg": "The account does not exist in the Cloud API. Use /register API to create an account first.",     "fbtrace_id": "A6fN7xoCSrj2t1dW_Qvv5d0" } 

In my Flutter project (VSCode), the Debug Console also shows:

"flutter: {error: {message: The account is not registered, type: OAuthException, code: 133010, error_subcode: 2593006, is_transient: false,..."

Despite my best efforts, I've reached a dead end... help me please.

I've tried everything I can think of: checking all the API settings, sending messages to different numbers, reviewing the documentation, and searching the internet for similar issues. But nothing seems to work. I just need to get the tests running again.