How to run start scripts from package.json?

npm - The main scripts such as start, stop, restart, install, version or test do not require run command. These scripts and some other are described in npm documentation.

npm start

The others need run command before the script name as was pointed by David.

npm run start:dev

Yarn - You can also use yarn and in that case you do not have to specify run.

yarn start

yarn start:dev

For most custom npm scripts you need to add run before script name npm run start:dev