Root Cause....

1. Causes (causal factors) have to be specific behaviors or conditions. A legitimate causal factor would be something like "training did not include how to self-identify fitness for duty degradation. " As long as you say exactly what is lacking and you can show that the lacking did, in fact, have an adverse effect on that which you allege it to be a cause of, it would work as a causal factor. (Categories don't cause anything. Causes do. Categories are just labels for causes.) (You can never show that you have addressed a category, but you can show that you have addressed a cause.)
2. Ask, "What are the factors that directly resulted in the nature, the magnitude, the location, and the timing of that specific lack?" The answer will give you some insight as to whether the lack should be regarded as a root cause or whether you need to dig deeper.
3. No training problem can ever be a root cause, because training problems always have deeper more important underlying factors.
4. "Lack of..." is an example of a negative cause, i.e., a cause consisting of the absence or insufficiency of something. It is an awkward propertey of the unverse that any effect can have an infinite number of negative causes and there is no known way to show that you ever got all of the important ones.