Issue of Magento 2 Layered Navigation on Magento 2.2.6 CE: Class Magento\Elasticsearch\Model\Adapter\FieldMapperInterface does not exist

Issue:

About the issue, our developer has checked and found the root of it is that the class Elasticsearch is not in Magento 2.2.x Community version; it is only in the Enterprise version.

Hence, to fix the issue, we would suggest that you should try to comment the code in the file etc/di.xml

<!-- Compatible with elasticsearch v2.4 && M2.1.8 -->
<type name="Magento\Elasticsearch\SearchAdapter\Filter\Builder\Term">
    <plugin name="bss-ln-convert-condition" type="Bss\LayerNavigation\Plugin\TermFilterBuilder" sortOrder="1" />
</type>

and then remove this file

Bss\LayerNavigation\Plugin\TermFilterBuilder.php 

Please check and follow our guide to fix the issue.

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