There are two flavors of table valued functions. One that is just a select statement and one that can have more rows than just a select statement.
This can not have a variable:
create function Func() returns table
as
return
select 10 as ColName
You have to do like this instead:
create function Func()
returns @T table(ColName int)
as
begin
declare @Var int
set @Var = 10
insert into @T(ColName) values (@Var)
return
end