When you need to capture packet, inspect network traffic using a proxy server on Android phone, you may required to install a SSL certificate in order to capture those HTTPS traffic. Download a certificate file in .crt, .pem format from the internet or export it from your computer then transfer it to your mobile phone. Tap to open the certificate files from a file manager on your Android phone, you will be prompted to enter your PIN. You need to have a PIN set otherwise by default you can not install any certificates to the Android device. The Name the certificate dialog pops up. Give the certificate name and press the OK button.
You should get “Certificate successfully installed”. You can verify that the certificate is installed by going to Settings > Security > Credential storage > Credential management > Trusted credentials > User. The settings may differ across various phones and tablets.