Sometimes checkouts will trigger warning messages that will appear in a yellow box above the check out field. These warnings need to be acknowledged before you will be able to continue checking items out.
Patron has outstanding fines
Item on hold for someone else
Item should be on the hold shelf waiting for someone else
Item already checked out to this patron
Item checked out to another patron
Item not for loan
Patron has too many things checked out
Item cannot be renewed
This can be overriden with the AllowRenewalLimitOverride system preference
Barcode not found