run in cmd

 npm install -g @angular/cli

and then

( open in Windows 10) Control Panel\All Control Panel Items\System or accordance with the figure

step 1:

https://i.stack.imgur.com/YAFVZ.jpg

step 2 :

https://i.stack.imgur.com/wD6eZ.jpg

step3:

https://i.stack.imgur.com/B28gO.jpg

step4:

https://i.stack.imgur.com/C0TDw.jpg

step5: add missing ng path

C:\\Users\\{your username}\\AppData\\Roaming\\npm
C:\\Users\\{yourusername}\\AppData\\Roaming\\npm\\node_modules\\@angular\\cli\\bin

https://i.stack.imgur.com/FUPTb.jpg

Here is new environment variable that you need add: C:\\Users\\AD\\AppData\\Roaming\\npm\\node_modules\\@angular\\cli\\bin

Finally, restart all opened command prompts and try again.