Probably a glitch, as the feedback system suffers from this a lot, maybe one of the reasons eBay allow us 60 days in which to leave it.
Are you using the useless app? Try switching to a laptop / PC / browser but not IE, that may help.
There are different ways in which to try and leave it too, click on your feedback score then the link above feedback forum then to the right .... Leave feedback.
Otherwise it is just a case of .... please try again later.
One more thing , if you are trying to leave a negative feedback for a power seller, it will not allow you to do this, until 7 days pass, it's to give the busy seller time to resolve the problem with you first.
