Is a good question explored quite thoroughly here A discussion about bootloader locking/unlocking… AKA I want to relock my bootloader, should I?
The eosinstaller is quite new and one big downside (did I say already?) is that there is no documented way to get to get a log … anything we said here would be guesswork and you might get deeper in the mire if there is just something odd in your setup.
One thing to be sure of is the whole USB chain … so your best data cable (not a charging cable) perhaps a high capacity cable, certainly not a skinny one or one or one with damage, avoid USB hubs (including any hidden hub in your PC / laptop), plug the cable to the USB port soldered to the motherboard, looking at the machine manual if necessary. This is a very common cause of failure to identify the phone.
You use Windows, this may be your issue, please be sure to check and understand this [Solved] Windows does not see my phone in Fastboot mode … in an automated environment be sure to keep a Device manager window open as you proceed
- Good luck if you try to have a go again. I do think that it is unfortunate to have been given the impression that this would be dead easy.
Even to do the Windows drivers check (in order to avoid complications which might be difficult to understand) you would need to have done from the Install page > Requirements
Check that
adbandfastbootare enabled on your PC. If not you can find the setup instructions here or better, Widows user, Installing adb and fastboot on Windows PC.
Edit
This is something you really must understand before you flash your phone and at a minimum get a screenshot the Android SPL on your FairphoneOS before you go any further. Some help in understanding this here: (answered) Install /e/ on FP5 with newer Security Patches.