I am making this post to help others and not because I need help.
First and foremost, many people ask "Why am I required to prove that I don't have credit history if I don't even have an SSN?" and the answer is that in the US, many banks will allow you to obtain credit without an SSN or ITIN. You can read more about this here: https://www.creditkarma.com/advice/i/credit-card-without-a-social-security-number/
So, you can get credit without an SSN/ITIN and can have a credit file without an SSN or ITIN. This is why USCIS requires a letter from at least one of the agencies confirming that you do not have a credit file.
How do you get a letter stating you do not have a credit file?
Simply write a formal letter to one of the three agencies requesting your credit file. In the file state and place the following:
1) Explain that you are requesting your credit file and you do not have an SSN/ITIN. Explain that if no credit file is found, please return a letter stating so.
2) Provide any names used, date of birth, and any addresses you've lived at
We submitted an I-485 to Chicago on 3 June. Our credit card was charged 24 June, and we got the SMS response on 28 June (25 days later). To Chicago, I've seen a few reports listing 3 or 4 days, but most are showing 20 to 30 days for the SMS to arrive. Emails are never sent.
As far as I've seen on the forums, emailing lockbox support never elicits a response. Wait for 30 days from delivery and then talk to the USCIS customer support.