Let's stay you setup your BRM

You add Stripe - in TEST MODE.

you make some test rentals and dummy payments / cards.

THEN you put Stripe into LIVE mode.

You revisit the old booking - which has a test card.

You may see an error like this:

No such customer: cus_xxxxxxxxxxxxx; a similar object exists in test mode, but a live mode key was used to make this request., Stripe.createCard(), Reservation Reference : Jan0001-19, Stripe Transaction Code : tok_xxxxxxxxxxxxxx

It is a Stripe thing.  It simply means you can't mix and match customer with TEST cards and LIVE cards - for obvious reasons.

What to do next?

Simply start again with a new customer  (fake if you like) and add a Real card / payment to that customer.
you can always refund yourself after you new TEST in live.