Magento 2.3: Error Assigning Related Products

check the following tables in your DB:

catalog_product_link_attribute and catalog_product_link_type

If those are empty, you need to run the following SQL queries:

INSERT INTO catalog_product_link_attribute (product_link_attribute_id, link_type_id, product_link_attribute_code, data_type) VALUES
(1, 1, ‘position’, ‘int’),
(2, 4, ‘position’, ‘int’),
(3, 5, ‘position’, ‘int’),
(4, 3, ‘position’, ‘int’),
(5, 3, ‘qty’, ‘decimal’);

INSERT INTO catalog_product_link_type (link_type_id, code) VALUES
(1, ‘relation’),
(3, ‘super’),
(4, ‘up_sell’),
(5, ‘cross_sell’);