Good day, @Andy,
Let's make sure you're able to connect your Barclays account into QuickBooks Self-Employed.
The bank error OBA28 usually comes up if your account is not yet upgraded to open banking. To fix this, new bank connections for Barclays will require a new set of authentication process using the PINsentry app. Contact your bank to get instructions on how to get and use it.
Once you have it, go back to the Bank Accounts page and connect the account again. For more information about Banking in QuickBooks Self-Employed, see the following links:
Kindly update me on the result. I wank to ensure this is resolved for you, and I'll be right here if you need further help.