Trying to add a transaction in QBO for a Venmo payment, but I get a message pop up to select an account, but I have already selected my checking acct?

Let me help you accomplish your task in QuickBooks, @afriedman77-gmai.

Are you trying to record an expense in QuickBooks Online?

If so, let's make sure to select an account for the Payment account and choose an expense account in the CATEGORY section. That way, we can get rid of the error message. You can check the screenshot below as your reference:

If you've already checked the following information and still getting the same error, I'd recommend performing some basic troubleshooting steps. This might be a cause of too much-cached build-up in your current browser. 

To verify, open your account in a private window. That way, we can check if the issue is browser-related. Simply press the following shortcut keys to access this mode:

• Ctrl + Shift + N (Google Chrome).
• Ctrl + Shift + P (Firefox).
• Control + Option + P (Safari).

If you're able to successfully record the transaction, go back to your regular browser and clear its cache. You can also use another supported browser if the issue persists. 

In addition, you can check out this article for further help in entering and managing your expenses in QuickBooks: How to Record Expenses in QuickBooks Online.

Let me know if you need further assistance with this. I'm always around to provide answers and clarifications to your questions. Have a great day ahead and take care.