The error you've encountered may have caused by the incorrect selection of your tax code while writing down your expense transaction or due to your browser's cache.
There are times that the browser is full of frequently accessed pages, thus causing some unusual system responses. You can isolate this by logging into your QuickBooks Online (QBO) account using a private browser (incognito) and assigning the right tax code to your transaction.
After that, you can pull up an expense transaction report (i.e. Expenses by Supplier). This is to effectively monitor your business expenses and make sure you've recorded them accurately. Just go to the What you owe and Expenses and suppliers sections from the Report menu's Standard tab. From there, select the report that provides all the information you need.
In the meantime, I'm also adding this article to further guide you in managing your business expenses using QBO: Expenses and Suppliers. It includes topics about paying expenses and bills, supplier credits, and stock management to name a few.
Please know that you're always welcome to comment below if you have other concerns or follow-up inquiries about taxes and business expenses in QBO. I'm just around to help. Take care always.