Fastboot connection problem
You are mixing up adb and fastboot.

When a device is booted and visible via adb it can't be accessed via fastboot (and the other way around). Fastboot is only active while the bootloader is active.

Hence if your device is visible via adb devices you have to access it via adb. 

If you want to switch to fastboot mode have to boot boot to bootloader mode by executing adb reboot bootloader.

Then you can use fastboot.