To switch to a webview on an Android application, the application should enable setWebContentsDebuggingEnabled in their source code before building the apk file. User needs to ask their developer to rebuild their app.
Refer to document of Appium:
There is an additional step necessary within your app build, unfortunately. As described in the Android remote debugging docs it is necessary to set to true the setWebContentsDebuggingEnabled property on the android.webkit.WebView element.
How to enable WebViewContentDebugging in source code: How to debug webview remotely?