If you copy the Timepicker code from the MUI docs, you also need to install the lab package which contains the adapter code to integrate with date-fns. See the requirements here. For reference, you can see the package.json file from the live demo.

npm i @mui/lab

I had the same issue. What I did was installing date-fns as well

  npm i date-fns

I then stopped the localhost server and restart it again with npm start (I was using react)

npm start

sometimes it just needs this :

yarn add @mui/lab