Have you enabled ADB Debugging in Developer options together with USB Debugging?
Another command for checking Bootloader in fastboot mode is: fastboot oem get-bootinfo
Maybe Bootloader is closed and Bootloader Screen shows wrong information..
Have you tried unlock Bootloader again?
If you use Minimal ADB and Fastboot program on PC be sure you execute it with Administrator Permission.
Windows Start, Command Prompt - right click, Run as Administrator.
Re-doing again (it was in posts #406 and #408, showing that adb and fastboot commands work, i.e. that USB debugging was enabled, etc)
Now I did also your suggestion: fastboot oem get-bootinfo
- adb devices: WORKS
- adb reboot bootloader : WORKS
- fastboot devices: WORKS
- fastboot oem get-bootinfo: WORKS; says that bootloader is unlocked (just like fastbooot screen on the phone says, and booting screen saying that phone is un-trusted because of unlocked)
- fastboot flash recovery twrp-3.1.0-2-eva.img: NOT WORKING: says that command is not allowed (I tried with 3.1.0-1, -2 and -3, all are in \ADB folder, their IMG file names are correct, copy/pasted)
- fastboot oem device-info: Also NOT WORKING (not important)
Btw, it is the same P9, same two PCs (with the same ADB installation) I used a month ago when I unlocked the bootloader and first time successfully flashed twrp-3.1.0-1-eva.img
Now doing everything the same, itstarts flashing (sending 'recovery' (25220 KB)... OKAY [ 0.540s] writing 'recovery'...) and then says: FAILED (remote: Command not allowed)
ADB-FASTBOOOT commands from Command Prompt:
C:\adb>
adb reboot-bootloader
C:\adb>
fastboot devices
MWS0216729010044 fastboot
C:\adb>
fastboot oem get-bootinfo
...
(bootloader) unlocked
OKAY [ 0.010s]
finished. total time: 0.010s
C:\adb>
fastboot oem device-info
...
FAILED (remote: Command not allowed)
finished. total time: 0.010s
C:\adb>
fastboot flash recovery twrp-3.1.0-2-eva.img
target reported max download size of 471859200 bytes
sending 'recovery' (25220 KB)...
OKAY[ 0.540s]
writing 'recovery'...
FAILED(remote: Command not allowed)
finished. total time: 0.550s