Hi Guys,
I got it! 
The problem was at the NPM version number. Heroku tries to install the lowest version of NPM. Install in my case “npm ^ 6.0.0 installed”.
So I set in “Package.json”:
“Engines”: {
“Node”: “> = 12.22.1 <= 14.x.x”,
“NPM”: “7.19.0”
},
The version “NPM” is crucial.
Thanks anyway: @Richard_Nsama @DMehaffy @pjamieson
Best regards!