This post attempts to give a brief answer to problems similar to these:-
- Windows does not recognise my phone in Fastboot mode
- Windows:
Waiting for any devicein Fastboot mode - Error on step: connect-bootloader
Cannot execute command - no compatible device found
See also, more detailed, [HOWTO] Fairphone 3/3+ stuck in fastboot/bootloader mode with the Easy Installer on Windows
- For the response
Fastboot command not found
please see Post #2
Typical situation; you are installing /e/OS it started ok then suddenly next stage won’t work.
Step 1.
(If using eosinstaller Step 1. may seem impossible, move straight to Step 2.)
Boot the phone into Fastboot mode.
Run
fastboot devices
Aim: Check that Windows recognises the phone; the phone will self identify usually with its serial number and boot mode … you might see
123xyz4567 fastboot
Step 2.
If the device does not respond with its ID then we check for Windows drivers problem by:
open a Device manager window with the phone connected in Fastboot mode.
Does the phone show as a 
Problem device ?
If yes Windows update is expected to fix this. Windows versions may vary but attempt
- allow Windows to make an automatic online search … or
- with search with “Optional updates” enabled [1] … or
- Advanced search / Windows Help.
with the phone connected in Fastboot mode.
[1] GIF from Fairphone support:
Why does it happen?
Windows is inclined to install a basic driver for “normal” activities on the phone. Fastboot mode likely requires a more complex driver. The driver you need may have a name as “Combination driver” or a name where a different device or “Google” is mentioned; this is not a problem.
Looking ahead in your install … it may be a good idea to keep a Device manager window open as you proceed … if the process should fail at any point check the phone has not newly become a Problem device – again Windows update is expected to fix this if a different driver has become required say for fastbootd.
Pour les lecteurs français
/e/ support topic for OnePlus Installing drivers on Windows OS for OnePlus Users.
Another more thorough approach
Windows driver for repair for adb / fastboot Device doesn't mount on PC with USB - #8 by AnotherElk