When rejecting a time card, you may encounter the following warning: Time or time off cards were not rejected because they have already been approved or rejected, have been invoiced, or the accounting period is not open for time.
The reasons for failure are pretty self explanatory, but this document will show you how to check on each one. There is one reason missing from the warning dialog, which is if the card has already been through revenue recognition.
Make sure this isn't a timing issue. Perhaps somebody else already rejected the time card and this is simply a timing issue. If you refresh your screen, does the card go away?
Is the time card on an invoice? Look at the time card's history using the time browser to find out. If so, pull it off the invoice, then reject it.
Is the accounting period open for time? If not, open it.
Has the time card been through revenue recognition? Check the history section of the revenue recognition wizard. If so, reverse revenue recognition before rejecting the card.