Enterprise vault tab not showing in Exchange OWA

I will suggest start with the basic, check if you have missed any requirement.

https://www.veritas.com/support/en_US/doc/83825443-95977190-0/index

If the add-in was already showing up and now it is not then check what has changed recently.

Refer to the below tech notes

https://www.veritas.com/support/en_US/article.100048139

https://www.veritas.com/support/en_US/article.100048648