It's great to see a new face here in the Community. I hope your weekend is going well so far.
The Unapplied Cash Payment Income account is cash basis income from a customer's payments that have been received but not dedicated to any sales form. Example: Never declared the income on a sales form when you took the money in. Here are two common way to correct this:
You may need to apply the payment to an invoice dated prior to the date of the payment if the amount is incorrect within the Unapplied Cash Payment Income account.
Your Product or Service item on the invoice can be mapped to a bank account type instead of an income account. This will cause the payment to go into the Unapplied Cash Payment Income account. Here's a link that can provide some additional details on how to change the account type to an income account: Change the account for a product/service item.
It's my priority that you're able to get fix the opening balance on your customer's account. Please let me know if this helps. I want to make sure all of your concerns are addressed. Have a fantastic weekend!