You have inadequate space on the TempDB drive and the user database drive to rebuild a clustered index (yes, rebuilding requires the same free space as creating it), that's a pretty clear indication that you are low on disk space, when regular maintenance will run a drive out of space.
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