Field validation error on nodereference field during D6 to D7 migration
It doesn't look like there's support for migrating nodereference fields in D2D migrate. You'll need to write a custom migrate class. In general, I haven't had much luck with the D2D GUI; it's only useful for super-simple migrations. 

Writing a custom migration class (especially if you're using migrate_d2d) is really quite easy to do and a worthy exercise. Take a look at the migrate_d2d_example code. The nice thing is that migrate keeps track of the mapping between the old NIDs and the new so that you need only write one line of code in your constructor to ensure the reference field has the right value.

See: Migrate D6 node reference to D7 entityreference