Undefined constant SEARCH_API_SOLR_SCHEMA_VERSION in SolrBackendInterface

Same here.

Same here as well.

Same here as well.

This is not actually a problem in this module, but in Search API Autocomplete, and was already reported there: 🐛 Undefined constant Drupal\\search_api_solr\\SolrBackendInterface::SEARCH_API_SOLR_SCHEMA_VERSION Needs review . Please test the patch in that issue.

PS: It seems you (like many others – it's really easy to misinterpret) are confused by the "Issue tags" field. As the guidelines → state, they aren't meant for free text tags related to the issue, but only for specific categorization purposes, usually by module maintainers.
So, if you aren't sure your current usage is correct, please just leave the field empty.

Thanks 'drunken monkey' for pointing to correct Issue. It has fixed my issue.