How do I pay a customer who helped us with advice in good in kind (their request)?

Hi there, @jennyadinchristi.

Creating journal entry is the best way to move money from the expense account to the customer account. Since doing it doesn't work, I encourage seeking help from your accountant to offer some advice on correct accounts to use. They may also add suggestions on how to pay your customer for the goods they requested based on what's more suitable for your business and to your books.

I'm adding this article to know more about recording journal entries in QuickBooks Online: Create a journal entry.

Keep in touch if you need any more assistance with this, or there's something else I can do for you. I've got your back. Have a good day.