As the error clearly states, backup log is not allowed when the database is in simple recovery (which AdventureWorks is). If you want to plan with log backups, switch it to full recovery (alter database), take a full database backup and then you'll be able to make log backups.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability