Are you getting a specific error when you test / in runtime?

Can you screenshot how you have the "Create Payment" step mapped.

No error, the zap runs fine, it does create a payment in QBO but does not apply it to the invoice. It is left 'unapplied'. 2 screen shots attached

@MikeW it looks like you are mapping a "Line ID" into the "Invoice ID". Is there an Invoice ID you're able to grab from your previous step? Sometimes, it's just labeled as "ID". Let us know!

@MikeW I ran a test this morning on my Quickbooks account here is the payment set up that resulted in a paid invoice after successfully running the a test:

Customer Ref Value was pulled from the invoice creation in the step above.

If you leave out the Line amount Under line items the payment gets created and references the invoice but does not get applied. To apply the payment to the invoice you need a value in Line Amount.

You should also be able to remove your delay step. The invoice exists once the invoice number is returned to Zapier.

For Clarity the custom value in "Line Linked Invoice" is this:

Thanks for the detailed answer @BowTieBots!

@MikeW Does that help you with the trouble you were having? How are you getting on with that Zap?

@jesse @BowTieBots Thank you both! That seems to be it.