Hi,

On a demo instance (Berlin P0 HF4 & Calgary) the field is available for selection.
I did some comparison and it seems the field type in your instance is 'choice' for the 'os' field.
In a demo instance it is 'string'. The moment I modified the field type in a demo to choice I could no longer choose the field in a normalization record.
So change the field type to 'string' in your instance and you should be fine.
Alternatively you could ask SNC support to assist in this matter.

Regards,