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’);