Why do I keep on failng when trying to add an invoice to a credit note?

Hi there, @ matthiasherren.

Let's link a credit note to an invoice. First, we'll want to make sure you create a credit note, so you have something to match with your invoice. 

Here's how:

1. Go to the + New icon.
2. Select Credit note or Give credit.
3. Enter the customer’s name, then fill out the required fields: Date, Products/service items, Quantity, and Amount.
4. Select Save and close.

Then, you can apply it to the invoice. Here's how to manually apply a credit note to invoices:

1. Go to the + New icon.
2. Select Receive payment or Receive invoice payment.
3. Enter the customer’s name, then fill out the required fields. Leave the fields Payment method, Reference no, Deposit to, and Amount received blank.
4. In the Credits section, select the credit note. Then, in the Payments column, enter how much credit your customer wants to use. Make sure the total to pay (if any) is correct after applying the credit note.

To give you more insight into the process of applying for credit, visit this article: Create and apply credit notes or delayed credits in QuickBooks Online.

I'll be right here to help if you have any other questions. Just leave a reply below. Take care!