Did you receive an e-file acknowledgement? If yes, then the Form 9465 is in the IRS processing system. Six weeks ago covid-19 was changing the world as we know it. IRS has not been staffed for several weeks.
Form 9465 should tell you whether or not the return was accepted and will also give you a number identifying the acceptance. There are numerous cases of returns that seem to be lost in space at the IRS website right now. Assuming your client had a refund, you could have the client utilize the "Where's My Refund" section at irs.gov. You can also utilize the EF Center in Pro Series to verify whether or not the return was accepted and to retrieve the Submission ID.
If will be several months or more before they see any letter from IRS approving or denying that installment plan application.
Have them start making monthly payment via IRS Direct Pay until they get the letter.