Schematic input does not validate against the Schema

Schematic input does not validate against the Schema: {"version":"7.3.8","name":"first_app","routing":true,"style":"css"} Errors: Data path ".name" should match format "html-selector".

This is the angular error. Angular doesn't support "app", "my_app", "my/app".
To correct the above error you should write first-app instead of first_app, this is because angular doesn't support underscore in the project name. You should type ng new first-app to your terminal.

ng new first-app