If the recording was made before the automatic transcription option became available, you’ll need to re-process the recording first. Access the settings menu for the recording in one of the places the gear icon is available:
Via the “Search” menu option, check the Smart Search Speech processing. If it says “Not Processed,” then go ahead and click “Process.”
Wait a few minutes for the transcription process, and then attempt the import as described on the page entitled Importing Automatic Captions — Brand New Recordings.