I'd be glad to help share information about sales transactions in QuickBooks Online (QBO).
There are lots of possible reason why your taxable sales in the sales tax center is not equal to the taxable sales on the sales summary report. First is the report's date range. Please to ensure to select the correct date or period when running the report. If the report was pull up using the correct date range, you may have transactions that do now have taxes which will be recorded as sales but will not show as total sales in the sales tax report.
To verify this further, you can run a non-taxable sales report and check your transactions from there. Here's how:
On the QuickBooks homepage, click on Reports at the left pane.
Look for the Taxable Sales Detail in the search field.
Click Customize button at the upper right hand.
Adjust for the appropriate date range in the General section then choose Filter.
Go to the Filter section, check the box next to Customer Taxable.
Check the box next to Product/Service Taxable, select Non-taxable. The report shows as Taxable Sales Detail when you run the Non-Taxable Sales Report.