If a customer misses out on selecting 'Save later' and clicks on the close option, will he be able to view under 'Rewards'?

Yes, the reward would remain available in the 'My Reward' tab for 3 months and will remain valid for 90 days until it's opened. Once opened, they are valid for 30 days.

I need to help, I closed the app and didn't save the rewards what can I do? 

Nothing to worry, the reward would be available in the 'My Rewards' tab for 3 months. If not opened, it'll expire in 90 days.