I found the snippet below in the 3.2 user guide. It states that you can only select external antennas types. But in the user guide for 3.4, there isn't a restriction for antenna type. Either way, it doesn't elaborate on why you can not choose an internal type. I tried to change it as well and it's the same external only. I think it's to keep the template from failing by pushing the wrong config down to the APs.
3.2 User Guide

3.4 User Guide

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-Troy J.