You need to import the product firsts and then you can import the contracts.

An Odoo object is always related to many other objects (e.g., a product is linked to product categories, attributes, vendors, etc.). To import those relations, you need to import the records of the related object first from their own list menu.

You can do this using the name of the related record or its ID. The ID is expected when two records have the same name. In such a case, add ” / ID” at the end of the column title (e.g., for product attributes: Product Attributes / Attribute / ID).

Export and import data — Odoo 15.0 documentation