I'm new to ionic. My app works fine and builds properly on android but on iOS I have to fetch the "CDVUserAgentUtil.h' file not found" issue.
You are facing the webview problems, depending which plugin are you using in your App you will have this problems. The Apple only accepts Apps that are using the updated Webkview. Check if you are using some Cordova File System plugin and update it. You can also install the Ionic Webview plugin in your App to solve this.
Another good solution could be using Capacitor for building your Apps.