Generate unique SKU with WPUF product form - https://mahbub.me/create-unique-sku-wp-user-frontend-custom-field/

functions.php

<?php

function wpuf_unique_product_sku_notice( $error ) {
    if ( isset( $_POST['_sku'] ) ) {
        global $wpdb;

        $sku = $_POST['_sku'];

        $product_id = $wpdb->get_var( $wpdb->prepare( "SELECT post_id FROM $wpdb->postmeta WHERE meta_key='_sku' AND meta_value='%s' LIMIT 1", $sku ) );

        if ( $product_id ) {
            return __( 'SKU is not unique, you must enter unique SKU' );
        }
    }

    return '';
}

add_filter( 'wpuf_update_post_validate', 'wpuf_unique_product_sku_notice' );
add_filter( 'wpuf_add_post_validate', 'wpuf_unique_product_sku_notice' );

?>