You can read the Privacy Policy under Game Options -> Show Privacy Policy



First time the policy would have opened to either on Steam or if you don't have the Steam overlay in use it uses the web browser. You can get the policy pop-up again by deleting colossal.cgs file from your local Steam folder -> steamapps -> commom -> Cities_Skylines



I hope you get it to open, and if not please report this as a bug with information if the Steam overlay is in use and which web browser do you have! And if you tried to read the Policy from the options or getting the pop-up again.