Read up on the requirements for indexing a view, there are a lot of them.
This particular one (as Books Online would have shown) say that the view was not created using WITH SCHEMABINDING. However you won't be able to index that anyway because DISTINCT is not allowed in an indexed view, neither is UNION.
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