See Problem Solving - Lightspeed Invalid Foreign keys for Customer...

if you try to addTransaction to a reservation attached to a customer that doesn't exist in lightspeed you'll get this:

and in your event log, something like this:

When you transfer a transaction to lightspeed to be processed can also send over a customer.
But if you try to send over a customer ID that lightspeed does not recognise you will get this error.

"But wait! - how can this happen?"  I hear you say....

because even if you archive a customer you still don't get this error - lightspeed allows you to send in a transaction against an archived customer.
The answer is that you have probably merged customers in lightspeed:
if you merge customers in lightspeed then it does completely remove one of your customer ids forever and will see the above error message.

So what do you do about it?

you just need to make sure that your corresponding client record in BRM / RSM points to the right customer in lightspeed.
You find the customer record and just overtype the new client ID from lightspeed and then save.

Let's take a concrete example:
you have 2 LS customers which are actually the same:
  • Fred Jones - ID61
  • Fred Jones - ID205
in BRM / RSM...
  • you have a Fred Jones in BRM / BRM which is linked to lightspeed customer ID61
Then you do some tidying up and merge your two Fred Jones records in lightspeed.
it turns out this completely deletes the old record ID61 you need to fish out your Fred Jones in BRM / RSM and update its lightspeed ID to 205

See Also

sharing customer accounts and lightspeed