Hello, Esther.
I feel you, and it is indeed frustrating when you can't find the help resources you're looking for. So, I would like to take this opportunity to help you link the payment with the original invoice.
Can you elaborate more on your issue when attaching the payment to the invoice? Did you encounter any error messages? Or is the payment simply not appearing under the customer's account?
I'd appreciate more details about this, so I can lay down the appropriate steps and solutions.
In the meantime, let's do the following steps to help us take care of the invoice payment.
I take it that the payment has already been posted under the customer's account, but not linked with the original invoice. This typically appears as an unapplied payment record (or just a partial payment).
So, in this case, we would want to use the Receive payment function to link both the invoice and the payment entry. Here's how to do it:
- Click the + New button, then select Receive payment.
- Under Customer, select the customer's profile.
- Set the Payment date, Payment method, Reference no, and the account (where you deposited the funds).
- Under Outstanding Transactions, check the original invoice.
- Under Credits, check the Unapplied Payment entry.
- Review the payment entry, then click Save and close.

This links the payment entry, then closes the invoice.
If you need more help managing your invoice payments, we can take a look at this article: Record invoice payments in QuickBooks Online.
If you need to run reports for your sales, expenses, and other financial figures, I'll share this article: Run reports in QuickBooks Online.
I'm all ears and ready to help again if you have more concerns about receiving and linking payments. You can also add any other questions about other areas in QuickBooks Online. Always here to help.