I cannot install high sierra because i have legacy file vault users?

Hi,

Go to System Preferences > Security & Privacy > FileVault > turn off Legacy FileVault > when it's finished decrypting, turn it on again if you want to, and this will be FileVault 2, which is compatible with High Sierra. I don't remember for sure, but you may have to turn off Legacy FV for each user that has it turned on.

When googling it, I saw some cases where people had to turn FV on, then off. There were a couple of references to Sonos causing the message. By chance do you have it installed? Other people got the message and had to erase then install HS, which then worked.