I can't stress to you enough that Enable Oem need's to be on the "on position" so switch it on (allow the device to be oem unlocked)and leave it alone it's nothing to do with you not being able to get to fastboot. If you trip the frp on your phone you will not be pleased.
with that said with your phone switched on and USB DEBUGGING SWITCHED ON plug into your usb on computer open a command window and input the following command- adb reboot bootloader you will now see your blue light on the phone this means your sony is now ready for you to flash a custom recovery (assuming you have unlocked your bootloader ?)
sorry to take time to respond
I did what you said and followed the steps of the tutorial, but I found two problems, the mobile does not have root access and when I tried to install magisk I found the following error:
! Unable to unpack boot image
Failed!
! Installation failed