Flush rewrite rules on plugin activation to stop your CPT from returning 404 errors

wp-cpt-flush-rewrite-on-activation.php

<?php
function activate_acme_cpt() {
	cpt_function();
	/** CPT taxonomy functions can be added too */
	cpt_tax_function();

	global $wp_rewrite;
	$wp_rewrite->init();
	$wp_rewrite->flush_rules();
}

// Registers the plugin activation hook.
register_activation_hook( __FILE__, 'activate_acme_cpt' );