Well - Considering that you do not created any relationship, I mean your "wp_toolset_relationships" table does not contain any data.
*** Please make a FULL BACKUP of your database and website.***
please try to run following query on your database:
DROP TABLE IF EXISTS `wp_toolset_relationships`; CREATE TABLE `wp_toolset_relationships` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `slug` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `display_name_plural` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `display_name_singular` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `driver` varchar(50) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `parent_domain` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `parent_types` bigint(20) unsigned NOT NULL DEFAULT '0', `child_domain` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `child_types` bigint(20) unsigned NOT NULL DEFAULT '0', `intermediary_type` varchar(20) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `ownership` varchar(8) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT 'none', `cardinality_parent_max` int(10) NOT NULL DEFAULT '-1', `cardinality_parent_min` int(10) NOT NULL DEFAULT '0', `cardinality_child_max` int(10) NOT NULL DEFAULT '-1', `cardinality_child_min` int(10) NOT NULL DEFAULT '0', `is_distinct` tinyint(1) NOT NULL DEFAULT '0', `scope` longtext COLLATE utf8mb4_unicode_ci NOT NULL, `origin` varchar(50) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_name_parent` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_name_child` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_name_intermediary` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_label_parent_singular` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_label_child_singular` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_label_parent_plural` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `role_label_child_plural` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `needs_legacy_support` tinyint(1) NOT NULL DEFAULT '0', `is_active` tinyint(1) NOT NULL DEFAULT '0', `autodelete_intermediary` tinyint(1) NOT NULL DEFAULT '1', PRIMARY KEY (`id`), KEY `slug` (`slug`(191)), KEY `is_active` (`is_active`), KEY `needs_legacy_support` (`needs_legacy_support`), KEY `parent_type` (`parent_domain`,`parent_types`), KEY `child_type` (`child_domain`,`child_types`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
*** Please make a FULL BACKUP of your database and website.***
That's it. I hope the above solution will help you to resolve your issue. 🙂 Have a great weekend.