On your device, go into “Settings → System → About” and find the Build Number and tap on it 7 times to enable developer settings.
Now go to the new item “Settings → System → Developer options” and enable “usb-debugging” or sometime it’s called “Enable Android Debug Bridge”.
Hello,
I just bought a FP3 and am trying to install /e/ from a Linux Mint PC,
I am stuck at the part where the “Allow USB debugging?” should pop on the phone and doesn’t.
I installed adb and tried “adb devices” on the command tool and it says: AXXXX (phone serial) unauthorized.
Can someone help me ?
I must say I am a total newbie to Linux but eager to learn 
Thanks !
The post is a little “old”, but just for answer it:
Assuming that you already activated “developer mode” and “usb debugging”, and that the “allow usb debugging” pop up is not showing up, maybe in the status bar you should see a message “conected just for recharge” (or something like that). You have to click on it and choose “file transfer”.
On /e/ (Android) 10, it is a bit different:
Settings > About phone > find Build number , tap 7 times.
