'On' would be more natural here, but 'upon' would also be fine.
'Upon' has exactly the same meaning as 'on', but it is more formal and old-fashioned. With the exception of the classic opening line for fairytales 'Once upon a time..' and the very outdated exclamation 'Upon my word!', you can always replace 'upon' with 'on'. There are, however, thousands of situations where 'on' is appropriate and 'upon' is not.
My advice would be to forget about trying to use 'upon'. Recognise it if you come across it, but don't try to use it: you will never need to use this word. Just use 'on' in all cases.