"VAT" in cart and invoice missing

Hi,

That text comes from what you entered in your tax rate namekey.
So you first need to go in the menu System>Taxes in your HikaShop backend and click on the "manage tax rates" button.
There, you'll see the list of your tax rates. If the "namekey" of the tax rate is empty there, then click on the "new" button of the tax rates listing and create a new tax rate with the same rate and the text you want as namekey. Then save and go to the System>Taxes menu where you have your tax rules listing. There, edit your tax rules and select that new tax rate instead of the one with an empty namekey.
If you do have some text for the namekey of your tax rate, then that should mean that you used a text that corresponds to a translation key and that this translation key has an empty translation text in one of the language files loaded on the page. So in that case, you can either try to create another tax rate and use it instead of the one you already have (as mentioned above), or you can check your translations in the language files. It's likely to come from a translation override, as they can be configured under the Languages tab of the HikaShop configuration.