i think we need to change the procfile slightly
an i have installed yarn. that could be the case also.
so change the procfile file
web: npm run serve
web: npm start
and add one line in package.json
"serve": "lwc-services serve",
//for above line, when you build the app on heroku it was looking at this script. so i just ...
It would seem this is not possible. Note Heroku support says to go to StackExchange, so this is now the official answer.
Individual channels have advised me that IBM, Microsoft, or Amazon, may offer a working alternative, but Heroku is not.