The template for 'Ooportunity Product' does not include 'Opportunityid' as the default look up field in Ecxel Column.
Therefore, when you actually try to import the data, the error code says below;
[0x80040216] [The parent id is missing.]


It is very critical that the template for 'Opportunity Product' does not have 'Opportunityid' while 'Opportunity Product' entity is
a must item in the Opportunity Form.

Please consider to add 'Opportunityid' look up field in the template for the better experience.