Did you download it without installing it?
Look on the Launchpad and to see if there's a High Sierra installer there.
One possibility - High Sierra requires a Boot ROM (EFI firmware) which supports APFS. For most Macs (including MBP's), the Boot ROM would get updated (if necessary) as part of the OS install or update. It may be that, for whatever reason, it isn't getting updated.
Open the "System Information" app. It should open such that "Hardware Overview" will appear on the right. Look for "Boot ROM Version". The latest High Sierra installer may not install the latest Boot ROM as there were Security updates which updated the Boot ROM. However, if you have a Boot ROM with all numbers (something like "922.214.171.124.0"), that should be close enough for APFS. If you have a Boot ROM version that starts with "MBP101", post what version you have - it looks like "MBP101.00DA.B000" was current as of Oct. 2017 - something much lower (the "00DA" hex number) would indicate the Boot ROM hasn't been updated in a while.
You could also open the "Console" app and look for "/var/log" on the left, look for "install.log" in the middle and go to the very bottom of what appears on the right and see if there is any error messages. This type of log isn't easy to read so just look for something obvious around the time the installer was downloaded.
Boot ROM Version: MBP101.00EE.B0A