Solr works with a set of multiple configuration files.
Each Solr configuration file, has it's own purpose.
Therefore, in some cases, some publishers (CMS systems, etc), will chose to create their own structure for such Solr configuration files, such as, it is the case with Drupal, and maybe WordPress (WPSOLR), and others.
When uploading your solr configuration files, using your Opensolr Index Control Panel, it is, therefore, important to upload your files in a specific order:
Create and upload a .zip archive containing all your dependency config files such as .txt files, schema-extra.xml solrconfig-extra.xml, pretty much, everything except schema.xml and solrconfig.xml
Create and upload a .zip archive containing your schema.xml file since that defines all fields and uses references to the archive you uploaded before (that contains schema-extra.xml and others like that)
Create and upload a .zip archive containing your solrconfig.xml file, since this one will have references to field definitions inside your schema.xml and other dependency files.
So, basically, you should simply create those 3 archives and upload them separately, in this exact order, and then everything should work.
You can, of course automate this, by using the Automation REST API to upload your config files.