In QuickBooks Online, you can enter the account's beginning balance if the are no active transactions. To do that, you'll have to delete all transactions one at a time. Also, deleting them will affect your financial reports.
Here's how to delete the transactions:
To delete bank transactions from the banking page:
You might also want to ask for more help from your accountant. They might have other options for you so you can reconcile your Credit Card account.
Let us know if you have any questions. We'll be right here to help.