Most likely, your credentials were typed incorrectly and can't be verified against the server.
On your Print Center app window, look for the small lock icon (with an arrow around it) next to your print job. Click it to bring up the login screen again. Remember to use your Mount Sinai email in the "Name" field.
Alternatively, search for "Keychain Access" on your Mac, then search for "Library_Mobility." Delete those credentials and print again.