I’m having the same problem with LineageOS. I’m following this description: Install LineageOS on FP2 | LineageOS Wiki
I checked that adb and fastboot are installed.
~> adb reboot bootloader
~> fastboot devices
fcf9f660 fastboot
~> fastboot flash recovery twrp-3.5.0_9-0-FP2.img
target reported max download size of 536870912 bytes
sending ‘recovery’ (10406 KB)…
OKAY [ 0.329s]
writing ‘recovery’…
OKAY [ 0.251s]
finished. total time: 0.580s
I would say this looks good. But I simply can’t reboot into recovery by pressing Volume Down + On/Off. Each time, Android starts as usually and asks for my password.
I tried to reboot into recovery by using this command:
~> adb reboot recovery
but I get “no devices/emulators found”, in both cases, when the phone is off or on, doesn’t matter. I even did it now without the case, precisely pressing the buttons, but it doesn’t work.
Does anybody know why this is happens? I will not be able to install any OS if I don’t resolve that, I guess. Does this sound like a hardware problem to you?
It’s Volume Up + On/Off.
If you’re doing it while on android you need to have android debugging on in the dic:developeroptions .
Otherwise as a workaround you can use twrpwoflashing.
Ah, right, I I press Volume Up + On/Off, I get the same “no command” screen I posted yesterday, no wipe option, nothing. And after a while, my phone starts in Android as usually.
It probably only works directly after flashing it. As you’ve started your phone “normally” (according to your post above) it won’t do…
So I recommend your repeat (while you’re again in fastboot mode and fastboot connection is ok as mentioned above)
~> fastboot flash recovery twrp-3.5.0_9-0-FP2.img
then type
~> fastboot reboot
and press the vol+up key at the same time while you send the command (and keep it pressed for some seconds); in this special case the power on button is not needed
This should finally bring your FP2 into TWRP recovery. 
@Volker It worked! This was the solution! I prepared the command, hit enter and immediately pressed the Volume Up + On/Off buttons. No reaction. Without letting go the buttons, I disconnected the phone from my laptop and it worked. All the other steps were easy, Lineage is installed. Thank you so much! 
I’m happy to read that! Thanks for confirming the solution and have much fun using LineageOS!
Hi folks,
looks like with this topic I finally managed to come nearer to an answer to my bootloader problem.
My original intention is quite the same as Felicias. I’d like to change my FOS to /e/OS.
And roughly I managed to do the same as Felicia in Jan.
And now the BUT - for which I hope for some good ideas.
I also checked that adb and fastboot are installed on my desktop computer (Windows 10). The device is found in the first attempt
~> adb devices => brings result
~> adb reboot bootloader => starts FP2 (in fastboot mode?), LED flashing blue
=> BUT: no more device can be found
~> fastboot devices => no result
I assume, the problem is the default USB Mode. Default is set to “charging”, not to “file exchange”.
But I’m not able to change that, even with developer options set and USB debugging allowed. Everytime I change the default to “file exchange”, after reboot it’s set back to “charging”.
Any good ideas or suggestions for me?
Thanks a lot already in advanced
I would check if all driver are up to date by
Connect FP in fastboot to your PC
Go to Windows Update
Search for Updates
Additional (or similar)
Driver
Install whatever shows with Google
The USB Mode is AFAIK not relevant when you work in fastboot/bootloader, and its normal that it will always start with USB Charging and you will have to change to data transfer or whatever you need
While this should be true, I can attest of someone who had the same problem, on whose phone (FP2 under FPOOS 19.08.1) data transfer was not working, neither was fastboot or adb (and after all the basic and advanced troubleshooting I was suspecting a hardware issue), before they booted in safe mode, noticed a bug that was resetting the USB mode always to a different other random one after reboot, and changed it back to file exchange (in safe mode), then booted in fastboot and it was working again. I’m not saying it’s a similar case here (and probably it isn’t), and I’m only mentioning to say sometimes informatics are strange and illogical.
only sometimes? 