A guest can't see an upsell
Is the listing included? Check the Listings count for the upsell and make sure this listing is in it.
Is a Show or Hide rule blocking it? Open the upsell, go to Additional Setting and check the rules. Remember: Hide rules win over Show rules.
Does the rule depend on Booking Status? The status only changes to checked in when the guest taps I'm In in their Guest Guide. If they haven't, rules based on check-in won't kick in yet.
Is it outside Service Hours? Guests can only book inside the daily window you set.
A guest can't pay
Check that the listing is assigned to a connected Stripe account (Settings → Integrations → Payments).
Late check-out was paid, but the time didn't change
Open the reservation and check the check-out time and access code. If they didn't update, contact support from Settings → Contact Support with the guest name and order date.