There is an error on the time sheet such as overlapping hours. Please print a copy and take a closer look to catch the error. Or, he/she is attempting to submit 30 days after the end of the pay period. He/she will need to print and submit a paper copy of the time sheet and submit that for approval.
Occasionally, there is a problem with the approval queue that will need to be resolved. You will need to contact HR to get this fixed.