USCIS returned your filing without deciding it because the fee did not match what the form requires. This is a rejection, not a denial.
What happens next
Check the current fee on the USCIS fee calculator and refile with the correct payment.
Official USCIS wording
On September 6, 2024, we rejected your Form I-485, Application to Register Permanent Residence or Adjust Status, Receipt Number XXXXXXXXXXXXX, because you sent us the incorrect fee. We mailed your case back to you, including any supporting materials and the incorrect payment. Please follow the instructions to resubmit your case. If you have any questions,visit the USCIS Contact Center webpage at www.uscis.gov/contactcenter. If you move, go to www.uscis.gov/addresschange to give us your new mailing address.