Unable to install a modified apk file

I want to monitor the https traffic of an Android app downloaded from google play. This app can only be run at Android N and above. So I use Charles proxy and follow [this document](https://www.charlesproxy.com/documentation/using-charles/ssl-certificates/) to modify the apk file. Here's what I have done: 1. Download the app from google play. 2. Use [apk extractor](https://play.google.com/store/apps/details?id=com.ext.ui&hl=en) to get the apk file of the app. 3. Run apktool d the_app.apk to extract the apk file. 4. Modify res/xml/network_security_config.xml and AndroidManifest.xml based on the description of [Charles Proxy document](https://www.charlesproxy.com/documentation/using-charles/ssl-certificates/) . 5. Run apktool b the_app to package the files back to an apk file. 6. Install the apk on my Android phone, whose version is 8. 7. My phone says "App not installed". Why can't I install the modified apk on my phone? I've disabled Google Play Protect on my phone. The apktool is installed by following the [description here](https://ibotpeaches.github.io/Apktool/install/) . Is there anything I missed?