Good coaches bring out the best in the teams they work with. An agile coach needs a broad set of skills. First and most importantly, a good agile coach is highly knowledgeable about agile, and ideally multiple agile frameworks. This means training and experience in any aspect of agile can be useful for an agile coach.
Recommended courses are a great way to expand your agile expertise.
Optional courses are less specific to this particular role, but will still provide value.