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
Example situation; you are installing /e/OS it started ok then suddenly next stage won’t work.
Step 1.
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.
[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.