Cannot Check Cluster Index Sql Server
Your script is correct, and if you get the error 


  The object 'postTB' does not exist in database 'cwpros' or is invalid
  for this operation.


this means some of the following: 


you created your table in other db than cwpros
someone dropped your table
your default schema is not dbo and you have anothe object with the
same name in your schema, but it's not a table


So first of all make sure that the object does exist in your database:

select *
from sys.objects
where name = 'postTB'