Sitecore Solr setup: Document is missing mandatory uniqueKey field: id

While reconfiguring Sitecore (8.2u5) to use Solr (6.6.1) instead of Lucene, I came across the following error:

Document is missing mandatory uniqueKey field: id

In full:

Job started: Index_Update_IndexName=sitecore_master_index|#Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> SolrNet.Exceptions.SolrConnectionException: <?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader"><int name="status">400</int><int name="QTime">1</int></lst><lst name="error"><lst name="metadata"><str name="error-class">org.apache.solr.common.SolrException</str><str name="root-error-class">org.apache.solr.common.SolrException</str></lst><str name="msg">Document is missing mandatory uniqueKey field: id</str><int name="code">400</int></lst>
</response>
 ---> System.Net.WebException: The remote server returned an error: (400) Bad Request.
 at System.Net.HttpWebRequest.GetResponse()
 at HttpWebAdapters.Adapters.HttpWebRequestAdapter.GetResponse()
 at SolrNet.Impl.SolrConnection.GetResponse(IHttpWebRequest request)
 at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
 --- End of inner exception stack trace ---
 at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
 at SolrNet.Impl.SolrConnection.Post(String relativeUrl, String s)

Here’s what to check.

Does your Solr core index config directory have a file called managed-schema? If so, delete this file and reload the core. Solr will be ignoring any changes you’re making to schema.xml and using managed-schema instead. Deleting this file and reloading the core will pick up your latest version of schema.xml

Delete this file