I want to add my bank account/card details to my expenses so my accountant and I can see what I've spent for the business for tax. When I try it says there's already bank

Welcome to the Community, originpestmanage.

You can add multiple banks or credit card accounts in QuickBooks Online. If you're getting a message that there's already a connected bank account, it could be you've already added your bank account in QuickBooks Online. You can check your connected bank or credit cards under the Banking page or in the Chart of accounts. 

To check connected bank or credit card accounts:

1. Click on Banking/Transactions tab.
2. Or click on the Accounting tab.
3. Click Chart of accounts.
4. Under Chart of accounts, you'll see all accounts in your QuickBooks Online account.

If you have added your bank account in QuickBooks Online, you can record and track your expenses and allocate them to your bank account. Here's a handy link you can refer to about Connecting bank and credit card accounts to QuickBooks Online.