You have either selected a tax liability account on a transaction where it's not allowed, or haven't specified a tax rate along with it.

Hi lvn_endaya,

Thanks for reaching out here.  It's essential that you're able to record your transactions seamlessly, in QuickBooks Online. 

In this situation, it's best to speak with your accountant for the best course of action when selecting the appropriate accounts associated with sales tax.  If you don't have an accountant, we can help you locate a ProAdvisor in your local area.

If you have other questions, feel free to reach back out.  We would be happy to help!