If a user has access to tax profiles in an expense item, a “none” option is available in the dropdown menu. If you want to remove this option, it can be turned off in the Features.
Navigate to Settings > Company > Features
Click the Edit button and scroll to the Expense feature Disable "none" as an option for tax profiles.
Check the box to remove the “none” option.
Scroll to the bottom of the features and click the Apply button.