Making a credit note from a supplier is currently unavailable in QuickBooks Online. Since the system will ask you to choose a customer, as a workaround, you might want to add the supplier as a customer with a slightly different name so you can choose them when creating a credit note.
However, if you don't want to add them as a customer, you can enter a supplier credit. Let me walk you through the process:
Click the + New in the left panel and choose Supplier credit.
In the Supplier drop-down, select your supplier.
Depending on how you record purchases with this supplier, enter the Category details or Item details. Usually, this is the Category, Product, or Service you’re getting credit for.
Hit Save and close.
For your references, you can go through the resources below. These will provide you ways on how to manage credits as well as steps on how to link these transactions to their bills: