decktalk check reports it when it judges the project’s files before a build.
Pass --allow SCRIPT_VOICE_UNKNOWN to carry on past it, and --fail-on to choose which severity fails a run.
What to do
Declare the voice as a[voices.NAME] table in decktalk.toml, correct the name in the direction, or reword a line that opens with the word voice.
Troubleshooting says more under A voice direction that names no voice.