I've read all the threads about this error without success. I think I've done all I could do myself.
Did you enter your billing information?
Yes. I can see billing information in my Production MCC account. I've seen AdWords API T&C and accepted it.
Are you having trouble entering your billing information?
No. I've added credit card successfully. It was charged for a few cents (like when you link a card to Google Play) from Google Inc. So I'm sure the card was added successfully.
Are you using an unapproved developer token against a production AdWords account?
No. I'm using developer token from Production MCC against the test client account under Test MCC.
Still receiving INCOMPLETE_SIGNUP error after completing the above steps?
Yes! I've submitted a form to AdWords API Compliance and received an answer with same steps. So the tals way was to use the developer forum.
My Client under Test MCC: 861-475-5602
In the Developers Console I've created a "web application" logged in with the email of my Test MCC.
When a logged in as Test MCC owner account I go to my own web application and go through OAuth2 flow, grant access to my application.
Then I setup the client library in my web-server application with:
Developer Token: Developer token from my Production MCC.
Client Customer ID: Client under Test MCC (861-475-5602).
OAuth2 Client ID — From the app in Developers Console as described above.
OAuth2 Client Secret — Secret of the app.
OAuth2 Token: The one from auth flow with Test MCC owner.
In debug mode I see Authorization header with the right token and SOAP body contains clientCustomerId. So configs are fine.
After all I receive INCOMPLETE_SIGNUP with any api request.