Declaring a scalar variable that its a table valued parameter?
The problem is here:

WHERE   (tsl.TransmittalSiteLeadFacility = @facilities


You are trying to compare a column on the left to a table on the right. This is like saying:

FROM sys.objects
WHERE object_id = sys.columns


...which obviously makes no sense.

That where clause doesn't belong there at all. You've already ensured that the facilities listed in the incoming table-valued parameter are matched to TransmittalSheet by using it in the JOIN clause. There is no need to filter on that match again.

My guess after several rounds of teeth-pulling is you want this:

CREATE PROCEDURE dbo.GetImpDocsThree
  @facilities dbo.Facilities ReadOnly,
  @facilitydivisionID tinyint = 6
AS
BEGIN
  SET NOCOUNT ON; 

  ;WITH f AS
  (
    SELECT FacilityID FROM @faclities
    UNION ALL SELECT @facilitydivisionID
  ) 
  SELECT  di.DocumentNumber, di.DocumentVersion, di.DocumentTitle, 
    ts.TransmittalNumber, ts.TransmittalVersion,
    FacilityImpDate = COALESCE(ts.TransmittalSiteLeadImpDate, ts.TransmittalImpDate)
   FROM dbo.DocumentInfo AS di
   INNER JOIN dbo.TransmittalSheet AS ts 
     ON di.DocumentTransmittalImp = ts.TransmittalID
   INNER JOIN f 
     ON tsl.TransmittalSiteLeadFacility = f.FacilityID;
END
GO


Or just insert 6 into the TVP up front too, and ditch the additional single facility variable altogether.