Hello everyone,
Sorry for the low quality screenshots, it is because of the file upload limit.
Long story short:
How to use the Variant entity in DIEF module?
There is a mandatory field named RetailVariantId. I can not make it auto generated, or give AX the next number in number sequence manually. (I found a thread about RetailVariantID here, thanks to Pavel C. Ondreicsik but, this information could not solve the issue I am facing with.)
Either way it tells me:

This is the staging-target mapping:

Entity structure:

The actual long story :)
I am trying to do a complete run of importing products by using the standart DIEF module.
First I tried to create a product master with using the Product Master entity but, AX kept giving me errors about number sequences, I worked on it pretty hard. Could not solve the issue, so I tried my chance on Product entity and succeeded.
Now I have a released product master with no released variants (No existing InventDimCombination for this item)
So, now I am trying to define variants with using the variant entity. And struggling with the problem written at the beginning of this post.
Environment
Application version: AX 2012 R3 CU8.
With no development at all. This environment is brand new.
Thanks.