The message "The 'NF' attribute is invalid - The value '' is invalid according to its datatype 'http://www.fei.org/Result:stNOCCode'" appears if there is a team in the team competition which does not have 3-letter country code for the given team.

Update the shortening field by selecting the team under clubs and add the country abbreviation.
If its a competion not based on 'Nations' just select the country of one of the competitors.