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I know the reason why you're unable to link it. Let me share it with you.
In using online banking, we're unable to connect both a parent account and its subaccounts. What you'll need to do is determine how your bank handles the transactions.
If the subaccounts are synched and you want to link the parent account, you'll need to disconnect the subaccounts. This is also advisable if your bank downloads the transactions from one account only.
I have some articles you can check. These will explain further how to setup subaccounts and how to properly connect American Express account:
About bank or credit card subaccount setup
Connect your American Express Business account to QuickBooks Online
Once everything's good, you can now categorize and match the bank transactions in QuickBooks Online.
Post again here if you need anything else. Have a great day!