Existing bank account won't allow posting of bill payments

Thanks for posting the Community space, @norman-smith.

You can make payment through the vendor's profile and select the correct bank account to post the bill payments. I'll guide you how:

1. Click Expenses on the left side menu.
2. Choose Vendors.
3. Select the vendor name.
4. In the Action column, select Make payment.
5. From the Bank/Credit account drop-down, select the bank.
6. Enter the necessary information.
7. Click Save and close.

In case you want to pay your bills directly and hassle-free, you can check this article for reference: What is Online Bill Pay?

However, if it's the bank, you can manually upload the expenses, here's how:

1. Go to the Banking or Transactions menu. 
2. Select File Upload.
3. Click Browse and choose the.CSV file, then select Next.
4. Select the account from the QuickBooks Account drop-down, then select Next.
5. Map the Date and a date format, then select the Description and Amount statement fields.
6. When ready, click Next.
7. Mark those transactions you want to import and click Next.
8. Choose Yes, then Let's go! when the Import completed notification appears.

For more detailed steps, click this article:  Import bank transactions using Excel CSV files.

Once done, you can categorize and match online bank transactions in QuickBooks Online.

Keep me posted on how things work on your end. I am always here to help you need anything else. Have a great day!