Restart AFP service in High Sierra

serveradmin is still available in macOS High Sierra. Server.app has to be installed though and launched at least once to start its set-up.

host:~ $user: which serveradmin
/Applications/Server.app/Contents/ServerRoot/usr/sbin/serveradmin

host:~ $user: sudo serveradmin fullstatus afp
afp:state = "STOPPED"
afp:readWriteSettingsVersion = 1
afp:logging = "NO"
afp:servicePortsRestrictionInfo = _empty_array
afp:startedTime = ""
afp:guestAccess = "NO"
afp:logPaths:errorLog = "/Library/Logs/AppleFileService/AppleFileServiceError.log"
afp:logPaths:accessLog = "/Library/Logs/AppleFileService/AppleFileServiceAccess.log"
afp:failoverState = "NIFailoverNotConfigured"
afp:servicePortsAreRestricted = "NO"
afp:setStateVersion = 2
afp:currentConnections = 0

AFP won't work with shared APFS volumes!

System Preferences -> Sharing would allow you to turn off all file sharing, then turn it on again.