@Bret Kean Welcome to Microsoft Q&A forum and thanks for reaching out here.
ML Runtime is not supported on below clusters. Does your cluster belong to any of these?
Were you able to see the ML section when you dropdown the Runtime selection menu? or is it greyed out?
In addition, I would suggest checking the cluster policy used. Check for Allow list policy and see if the ML runtimes are allowed or not.
Related doc: Manage cluster policies
Hope this info helps.
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