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.