I'm using an older Huawei phone. When I plug in to my Debian Stretch instance I get the USB computer connection popup. If this is set to MTP, PTP or "Connect to PC software" I found that "adb devices" will fail. However, if I set the connection option to:
Mount SD Card
I then could run "adb devices" and status changed from unauthorized to device.
Also I turned on:
USB debugging