Instlling exchange server 2010 client access roles

I am getting this error when trying to install client access role for my exchange server 2010
can anyone help
Summary: 3 item(s). 1 succeeded, 1 failed.
Elapsed time: 00:00:04


Preparing Setup
Completed

Elapsed Time: 00:00:00


Client Access Role
Failed

Error:
The following error was generated when "$error.Clear(); $InternalOwaUrl="https://" + $RoleFqdnOrName + "/owa"; new-OwaVirtualDirectory -DomainController $RoleDomainController -InternalUrl $InternalOwaUrl;" was run: "An error occurred while creating the IIS virtual directory 'IIS://DENCOFAM.denco-enterprises.com/W3SVC/1/ROOT/owa' on 'DENCOFAM'.".

An error occurred while creating the IIS virtual directory 'IIS://test.testserver.com/W3SVC/1/ROOT/owa' on 'test'.

ISAPI extension owaauth.dll in group MSExchangeClientAccess wasn't found and its status can't be changed. Please make sure this ISAPI extension (IIS Web service extension) is still listed in IIS Manager. If it's not, it may need to be added again.

Elapsed Time: 00:00:03


Finalizing Setup
Cancelled