Solr error creating core: fieldType [x] not found in the schema

Not sure whether it is the preferred way to go, but commenting out the solr.AddSchemaFieldsUpdateProcessorFactory section in config.xml seems to solve the issue.

<!--
<processor class="solr.AddSchemaFieldsUpdateProcessorFactory">
  <str name="defaultFieldType">strings</str>
  <lst name="typeMapping">
    <str name="valueClass">java.lang.Boolean</str>
    <str name="fieldType">booleans</str>
  </lst>
  <lst name="typeMapping">
    <str name="valueClass">java.util.Date</str>
    <str name="fieldType">tdates</str>
  </lst>
  <lst name="typeMapping">
    <str name="valueClass">java.lang.Long</str>
    <str name="valueClass">java.lang.Integer</str>
    <str name="fieldType">tlongs</str>
  </lst>
  <lst name="typeMapping">
    <str name="valueClass">java.lang.Number</str>
    <str name="fieldType">tdoubles</str>
  </lst>
</processor>
-->

<lst name="typeMapping">
    <str name="valueClass">java.lang.Boolean</str>
    <str name="fieldType">booleans</str>
</lst>

over here you need to correct "booleans" to "boolean".

<lst name="typeMapping">
    <str name="valueClass">java.lang.Boolean</str>
    <str name="fieldType">boolean</str>
</lst>

Then it will work..

Or

The alternate solution is to comment out the <updateRequestProcessorChain name="add-unknown-fields-to-the-schema"> section in solrconfig.xml.