How do we clear this error on an invoice? Were sorry, QuickBooks encountered an error while calculating tax. Try ... the product/service item and saving the form again."

Hello GMacD,

QuickBooks is made for business owners like you that aim to streamline their business. Dealing with error messages can be a time-consuming experience especially when you have deadlines to meet. Don't worry, I got your back, I'll be happy to assist you so you can continue doing what you love the most. 

Based on the error you're getting, it usually occurs when your invoice items don't have a tax code assigned to them, I suggest checking the items on your invoice to make sure you've assigned the appropriate tax codes. I also recommend making sure you have set up taxes correctly on your account. I trust that you have done that but if you need a reference on how to do that, here's a helpful article: Set up and use automated sales tax in QuickBooks Online. The article goes in depth showing you how to set up tax rate for your customers. 

If you're still having issues after checking the variables above, I recommend clearing cache and cookies.  You can fix minor display issues by following the steps in this article here: Clear Cache and Cookies. Give this a try and let me know how you make out. If you need further assistance, don't hesitate to get back to me in the comments below.

Otherwise, have a great rest of the week!