Algol Plus Ltd

1. Visit >Phone Orders>Settings>Common and turn on "Switch Customer"

2. Adjust Avatax settings,  set option "Cart Calculation" =  "Force full tax rate calculation"

3. AvaTax plugin doesn't work in admin context.  So you should edit files in this plugin a bit.

Please,  edit file   /class-wc-avatax.php   and comment lines  186,188

So you should get 

//        if ( ! is_admin() ) {
            $this->frontend = $this->load_class( '/src/frontend/class-wc-avatax-frontend.php', 'WC_AvaTax_Frontend' );
//     }