Angular CLI Error: The serve command requires to be run in an Angular project, but a project definition could not be found
Solution 1:
I was also getting this issue and solved by running below command.
ng update @angular/cli --migrate-only --from=<WhateverVersionYouAreCurrentlyOn>
e.g.
ng update @angular/cli --migrate-only --from=1.7.3
getting ref from here
Solution 2:
make sure that you are running the command in the application root folder..
Solution 3:
if you have downloaded a project,do in the project
npm install