no permissions fastboot - fastboot and adb not working

Try using sudo $(which fastboot)

for instance sudo $(which fastboot) devices

also sudo $(which fastboot) oem unlock

Found a better solution over here:
https://stackoverflow.com/a/28127944/1621927

Here is the comment:

Instead of forcing permissions via sudo each time you need to run fastboot, you can permanently fix the issue:

1. use lsusb to identify your device USB VendorID
2. configure udev to set proper permissions when your device is plugged in
3. profit!

As a bonus - it will be fixed for adb too.

For example, in my case (for 'Megafon SP-A20i') :

$ fastboot devices
no permissions  fastboot
$ sudo fastboot devices
[sudo] password for kaa: 
MedfieldA9055F28    fastboot
$ 

Let's fix:

First, we need to identify the device:

a) look for usb bus number (hack: I know the device is Intel-based one)

$ fastboot -l devices
no permissions         fastboot usb:1-1.2
$ lsusb |grep 001 |grep -i intel
Bus 001 Device 044: ID 8087:09ef Intel Corp. 
Bus 001 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
$ 

b) look for other Intel devices:

$ lsusb |grep 8087
Bus 002 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
Bus 001 Device 044: ID 8087:09ef Intel Corp. 
Bus 001 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub
$ 

Hubs are not smartphones definetely, so - USB vendorID we need is "8087".

Second, configure udev (you must replace "idVendor" value with yours) :

$ sudo sh -c "echo '# Megafon SP-A20i' >> /etc/udev/rules.d/51-android.rules"
$ sudo sh -c "echo 'SUBSYSTEM==\"usb\", ATTR{idVendor}==\"8087\", MODE=\"0666\", GROUP=\"plugdev\"' >> /etc/udev/rules.d/51-android.rules"
$ sudo service udev restart
udev stop/waiting
udev  start/running, process 1821
$ 

Third, re-plug your device to allow udev to perform it's magic.

Final check:

$ fastboot -l devices
MedfieldA9055F28       fastboot usb:1-1.2
$ adb devices
List of devices attached 

$ fastboot reboot
rebooting...

finished. total time: 0.253s
$ sleep 90
$ adb devices
List of devices attached 
MedfieldA9055F28    device

$ 

Voila!

My Solution

Thanks to elichai2 and this link and list I was able to solve my fastboot permission issue.

I have the Google Nexus 6P so I used the following for my Vendor ID:

Google - 18d1

51-android.rules

If you use the following command:

sudo sh -c "echo 'SUBSYSTEM==\"usb\", ATTR{idVendor}==\"8087\" ..."

copy it exactly. I was already root and tried to copy just the echo portion which gave me undesirable results. Basically it put the \" in the .rules file rather than interpreting the slash as the escape character.

I suggest using the following if you already root creating the .rules file (Use your vendor ID).

echo 'SUBSYSTEM=="usb", ATTR{idVendor}=="18d1", MODE="0666", GROUP="plugdev"' >> /etc/udev/rules.d/51-android.rules

Udev Reload and Trigger

I reloaded the udev rules and then issued the trigger command. I also unplugged and plugged the usb for good measure.

udevadm control --reload
udevadm trigger

Fastboot

As an unprivileged user I then ran:

fastboot devices

and it worked as desired. I hope this can be of use or help to anyone that experiences trouble with the permissions issue.