SSML Tag Errors

I am just learning about the use of the SSML tags to modify text-to-speech (I am trying o to slow down the narrator Stephen). I THINK I followed the guidelines on how to use prosody, but when I go to publish I receive the error "Storyline 60 can't convert this text to speech. Verify the SSML tags are correct and supported for the selected voice." Is SSML not supported for Stephen? (I thought I read other users were using him).

I just updated Storyline this morning, so I am assuming I have all of the proper bug fixes. Any advice would be appreciated. Here is my code (on my screen, "rate" is in red):

<speak><prosody rate="90%">Per the below chart, ground fault overvoltage at PHI is caused by inverter based D E Rs when the generation to load ratio exceeds 100% at the substation transformer and 67% for synchronous based machines. 
<p>Onboard D E R active or passive anti-islanding protection is not quick enough to protect against ground faults on the transmission substation bus when those ratios are exceeded. </prosody></speak>