Evertb showing up in windows explorer just means it is being detected as a storage device. This isn't the same as being detected as a fastboot device. What do you mean when you say boot unlocked? Toggled OEM unlocking on or actually performed the unlock in the bootloader via fastboot using the installer buttons?
What cable is being used?
Have you ensured to follow ALL instructions inclusive of installing the required driver if Windows hasn't done it itself?
You are making sure not to use an Incognito Window or have any "shields" enabled in your browser for the installer?