If you're VAT registered then you will need the add VAT to your invoices, even if the applicable VAT rate is zero which would make the VAT on the invoice due £nil.
If you can't see the VAT line on your invoice then please try the below:
Head over to the settings tab
Open up the VAT settings
Ensure that you have selected the button which marks your company as VAT registered
Click save VAT settings
If you still can't see VAT on your invoice after trying the above, please reach out to support.