Connect your phone to a PC/laptop and try the update from there. You'll need to install Samsung's Smart Switch on your PC first, allow it to install all the drivers, then connect your phone.
Other reasons why your phone won't update OTA (over the air) depends on your carrier, and who you bought your phone from.
For example - if you bought your phone from AT&T but are now running it on T-Mobile, you'll need to get back on the AT&T network to get your update.
What model is your phone (check the "settings" and "about phone")