This function will set the tax address for WooCommerce AvaTax when local_pickup is chosen

adjust_local_pickup_tax_address_for_avatax.php

<?php // only copy this line if needed
/**
 * This function will set the tax address for WooCommerce AvaTax when local_pickup is chosen
 *
 * @param  $address array	The orignal address
 * @return array		The original or modified address.
 */
function adjust_local_pickup_tax_address_for_avatax( $address ) {

	// get the chosen shipping method
	$chosen_shipping_method = WC()->session->get( 'chosen_shipping_methods' );
	$chosen_shipping_method = explode( ':', $chosen_shipping_method[0] );

	// if it's local pickup, change our address
	if ( 'local_pickup' === $chosen_shipping_method[0] ) {

		$address = array(
			'address_1'	=> '123 Main Street',
			'address_2'	=> '',
			'city'		=> 'Ocala',
			'state'		=> 'FL',
			'country'	=> 'US',
			'postcode'	=> '34480',
		);
	}

	return $address;
}
add_filter( 'wc_avatax_tax_destination_address', 'adjust_local_pickup_tax_address_for_avatax', 100 );