Try this in your phpmyadmin, let me know if you get the same error:

update products_restock_request_customers r set r.zen_customers_id = (SELECT customers_id from customers c where c.customers_email_address = r.email_address)

PS: If you have prefix for your zen table, you may have to use this:


update zen_products_restock_request_customers r set r.zen_customers_id = (SELECT customers_id from zen_customers c where c.customers_email_address = r.email_address)

// Replace zen_ by your prefix name (if you use a different one)