How do I add & link a debit card to checking account that is already linked to Quick Books?

Hello there, @steve137.

If the credit card is already linked to QuickBooks. You cannot link it to the checking account. However, you can manually upload the transactions in QuickBooks.

First, you'll need to pick a date range, here's how:

1. Click Accounting in the left panel.
2. Choose Chart of Accounts.
3. Locate the bank or credit card account you want to upload.
4. Tap View register.
5. Scroll to the bottom of the list.
6. Write down the date of the oldest transaction. In many cases, the oldest transaction is the opening balance.

Then, download the transactions from your bank:

1. Log in to your bank or credit card’s website.
2. Follow your bank’s instructions for how to download transactions to your computer. 
3. Ensure that the date range for the download is at least one day before your oldest transaction.
4. Download your transactions. 
5. Save the file somewhere you can easily find it, like your desktop.

For more detailed instruction, please read out this article: Manually upload transactions into QuickBooks Online.

Also, you can refer to this article if you have issues when manually upload bank transactions: Fix bank upload errors in QuickBooks Online.

Please let me know if you have any other questions. I’m always here to keep helping. Have a pleasant day ahead.