The letter is uploaded but it’s not submitted. I had the same issue when I did mine. The reason the red submit button doesn’t work is because you still have missing documents. Even if you are requesting a change in sponsor you will have to upload documents on those fields to be able to hit the red submit button. What I did was I uploaded the request to remove the sponsor in those other missing fields and once it said uploaded in all missing fields then I submitted by hitting the red button. Don’t worry about submitting the letter in multiple fields, once they remove the sponsor and they add the other one they will ask for documents for the other sponsor. So to recap, upload the letter to the fields saying missing beside them and once they say uploaded you should be able to hit the submit button. If you don’t do this they will never look at the letter. Make sure you upload the documents under the joint sponsor you want to remove. I spent 3 months trying to figure this out since I needed to remove a sponsor and what I am telling you is exactly what I did. If you have any other questions let me know as I am done with my nvc process I am just waiting for the appointment date at the embassy for my wife.