Hello Youn Ah Lee, 

Welcome to the Community page, 

Sound like you just don't have the setting turned on for it. 

Can you go to the main cog wheel>account and settings> expneses> under heading bills and expenses > make sure the one that says Make expenses and items billable is ticked on > save 

Go to the left-hand navigation bar and click into plus new expenses and the vat sections show up for you now. 

If you have any issues please come back to us and let us know.