Issue with Insert Text-to-Speech Dialog

Hello,

I am trying to insert a script in order to use one of the neural TTS voices. However, the Update button becomes inactive at a certain point when I enter the text and use minimal SSML tags. I checked my character count and I believe I am under the 10K limit. Any ideas why this is occurring? I would split the script into one or more smaller pieces, but I also want to use the Update Closed Captions option, and I believe that will then give me individual TTS files rather than one continuous file. I want to be able to generate the one TTS script and one .vtt file.

Thanks!

John