If you tried to update Angular version, It may cause a lot of dependency issues. This fix may help you.
Steps listed below are valid for Linux (Debian Family) OS.
Open terminal
Change directory to project folder
cd <path to project folder>
Remove All node Modules
rm -rf ./
node_modules
Install modules and Angular CLI back
npm install
npm install @angular/cli -g
ng update @angular/cli @angular/core --force
ng update --all --force
Now test
ng serve