If you usually get an error, such as: Unknown field... Or Missing field, and your schema.xml already contains those fields, make sure you disable the Schemaless mode in solrconfig.xml

Just head on to the Config Files Editor in your opensolr index control panel, and locate a snippet that looks like this:

class="ManagedIndexSchemaFactory"

According to the solr documentation, you can disable the ManagedIndexSchemaFactory as per the instructions below:

To disable dynamic schema REST APIs, use the following for :

<schemaFactory class="ClassicIndexSchemaFactory"/>

Also do not forget to remove the entire snippet regarding the ManagedIndexSchemaFactory, so that you won't accidentally use both.