product_variant_csv_import: insert or update on table "product_attribute_line" violates foreign key constraint

[alex9434]

I am working on Odoo8. Unfortunately the module does not have instructions. Therefore I have looked at the encl. examples in
https://github.com/akretion/odoo-usability/tree/8.0/product_variant_csv_import/sample_csv
and have tried to import them:
product.attribute.csv Imports without problems
product.attribute.value.csv Imports without problems

but with product.template.csv I get the following error message:

insert or update on table "product_attribute_line" violates foreign key constraint "product_attribute_line_attribute_id_fkey" DETAIL: Key (attribute_id)=(12) is not present in table "product_attribute". between rows 2 and 9 (1 more)
insert or update on table "product_attribute_line" violates foreign key constraint "product_attribute_line_attribute_id_fkey" DETAIL: Key (attribute_id)=(12) is not present in table "product_attribute". between rows 10 and 17

What is causing the error?