failed to run Angular application getting error : Schema validation failed

This is due to npm audit fix installing a newer version of the @angular-devkit/build-angular package, specifically the 0.800.x versions, which has breaking changes from the 0.13.9 version. Thus, running the 0.800 version of this package breaks the angular-cli when used with Angular 7.

To fix this issue install the older stable package version with:

npm install --save-dev @angular-devkit/[email protected]

OK I did some testing and on my side (check out my comment in the question) it seems that the npm audit fix breaks the dependencies and causes ng serve to yield on that error. It seems to have to do with a dependency and I hope they'll fix it soon...

Even I ran into the same problem. As @Ardzii suggested the issue is with 'npm audit fix' command. I cloned a new repo and installed the node packages again without running the npm audit command and now it works fine.