TypeError: Cannot read property 'flags' of undefined

Removing @angular/cli should work for you

if not, change Typescript version to ~2.6.2.

@angular/cli 1.7.3 works with Typescript ~2.6.2 only.

Installing angular 9.0.0 solved the problem for me

npm install @angular/[email protected] -g

it was breaking for angular 9.1.5