SQL parses your script and finds that you are attempting to create it in two different places, and so raises an error, its better to create the table in the beginning with common columns and add the specific columns in the if--else based on the condition

Declare @Version int;

select @version=(SUBSTRING(CONVERT(VARCHAR(50),SERVERPROPERTY('productversion')), 1,2))

IF OBJECT_ID('tempdb..#checkresults') IS NOT NULL

DROP TABLE #checkresults

create table #checkresults

(

[Error] [int] NULL,

[Level] [int] NULL,

[State] [int] NULL,

[MessageText] [varchar](650) NULL,

[RepairLevel] [varchar](150) NULL,

[Status] [int] NULL,

[DbId] [int] NULL,

--[DbFragId] [int] NULL,

[ObjectId] [int] NULL,

[IndexId] [int] NULL,

[PartitionId] [int] NULL,

[AllocUnitId] [int] NULL,

--[RidDbId] [int] NULL,

--[RidPruId] [int] NULL,

[File] [int] NULL,

[Page] [int] NULL,

[Slot] [int] NULL,

--[RefDbId] [int] NULL,

--[RefPruId] [int] NULL,

[RefFile] [int] NULL,

[RefPage] [int] NULL,

[RefSlot] [int] NULL,

[Allocation] [int] NULL )

if (@version >=11)

begin

alter table #checkresults add

[DbFragId] [int] NULL,

[RidDbId] [int] NULL,

[RidPruId] [int] NULL,

[RefDbId] [int] NULL,

[RefPruId] [int] NULL

end

drop table #checkresults