I might be wrong but is it because you don't have a text to speech engine installed? Grapheneos doesn't come with one pre-installed like stock does.
"Speech recognition and synthesis from Google " - search on Aurora store. If you plan to restrict network access, you may need go temporarily allow network access to download content to your phone before restricting it again.
An open source alternative is RHVoice (available on play store/Aurora, F-Droid, and github). Not as pretty as google's but could get the job done.