Why am I getting an error saying it cannot find Azure.Search.Documents ?

Question

Message: Creating or updating index 'docsearch-dev' failed with the following error: Could not load file or assembly 'Azure.Search.Documents, Version=11.6.0.0, Culture=neutral, PublicKeyToken=92742159e12e44c8' or one of its dependencies. The system cannot find the file specified. Index fields: sys_type (Edm.String), sys_id (Edm.String), sys_site (Edm.String), sys_created (Edm.DateTimeOffset), sys_culture (Edm.String), sys_index (Edm.String), sys_content (Edm.String), nodeid (Edm.Int32), nodealiaspath (Edm.String), nodelinkednodeid (Edm.Int32), documentid (Edm.Int32), classname (Edm.String), documentpagetitle (Edm.String), skushortdescription (Edm.String), documentpublishto (Edm.DateTimeOffset), nodeowner (Edm.Int32), skuname (Edm.String), documentpagekeywords (Edm.String), nodename (Edm.String), nodeguid (Edm.String), documenttags (Edm.String), documentpublishfrom (Edm.DateTimeOffset), nodecustomdata (Edm.String), documentnodeid (Edm.Int32), nodelevel (Edm.Int32), documentforeignkeyvalue (Edm.Int32), nodeaclid (Edm.Int32), documentculture (Edm.String), nodeorder (Edm.Int32), nodeclassid (Edm.Int32), nodealias (Edm.String), documenttaggroupid (Edm.Int32), documentlastpublished (Edm.DateTimeOffset), issecurednode (Edm.Boolean), documentsearchexcluded (Edm.Boolean), nodesiteid (Edm.Int32), nodeparentid (Edm.Int32), documentname (Edm.String), documentcreatedwhen (Edm.DateTimeOffset), documentpagedescription (Edm.String), nodeskuid (Edm.Int32), title (Edm.String), documentcategories (Collection(Edm.String)), documentcategoryids (Collection(Edm.String)), nodealiaspathprefixes (Collection(Edm.String)), primarytopic (Edm.String), thumbnailimage (Edm.String), summary (Edm.String), pressreleasenodeid (Edm.Int32), publishdate (Edm.DateTimeOffset), shortsummary (Edm.String), alternateurl (Edm.String). Exception type: System.InvalidOperationException Stack trace: at CMS.Search.Azure.SearchServiceManager.CreateOrUpdateIndex(Index index) at CMS.Search.Azure.SearchTaskEngine.UpdateIndex(SearchIndexInfo indexInfo, IEnumerable1 searchables, Func3 transformSearchableToIndexAction, Index existingIndex) at CMS.Search.Azure.SearchTaskEngine.Rebuild(SearchIndexInfo indexInfo) at CMS.Search.Azure.SearchTaskEngine.ExecuteRebuildTask(SearchTaskAzureInfo task) at CMS.Search.Azure.SearchTaskExecutor.Execute(TaskInfo task) Message: Could not load file or assembly 'Azure.Search.Documents, Version=11.6.0.0, Culture=neutral, PublicKeyToken=92742159e12e44c8' or one of its dependencies. The system cannot find the file specified. Exception type: System.IO.FileNotFoundException Stack trace: at Bridgespan.CMS.AzureSearch.SynonymModule.AddSynonymMap(Object sender, CreateOrUpdateIndexEventArgs e) at CMS.Base.AbstractHandler.CallEventHandler[TArgs] at CMS.Base.AbstractHandler.Raise[TArgs] at CMS.Base.SimpleHandler2.RaiseExecute(TArgs e) at CMS.Base.SimpleHandler2.StartEvent(TArgs e) at CMS.Search.Azure.SearchServiceManager.CreateOrUpdateIndex(Index index)