The most common cause of the issue you describe are unmet requirements. One of them is the framework on which the EU VAT Assistant is based. It’s a plugin called Aelia Foundation Classes (AFC, for short).
The EU VAT Assistant is designed to asking to install the AFC plugin when it doesn’t find it, showing a message similar to the following: https://prntscr.com/f6be2d. If the message doesn’t appear, it means that something on the site is hiding it. Some plugins tend to hide those admin messages, either due to “invasive” styles or by design, as an attempt to “de-clutter” the Admin section, but by doing so they prevent Administrators from reviewing conditions that require an action from their side.
Even if you don’t see the message prompting to install the AFC, you can still download and install the plugin manually. You can get the AFC plugin from here: wc-aelia-foundation-classes.zip. Simply upload and activate it (no configuration required), and the EU VAT Assistant menu should appear automatically. The AFC framework will keep itself up to date automatically, so you can forget about it after you completed the installation.