In the Wiki, ( https://wiki.dovecot.org/Plugins/FTS/Solr ), it would nice to stipulate to the reader  to type the command :

sudo -u solr /opt/solr/bin/solr create -c dovecot # to create the dovecot instance

before updating the schema.xml .

Also,  schema.xml is in /opt/solr/server/solr/dovecot/conf for archlinux users

Additionaly, the url is http://(solr_server):8983/solr/dovecot/ (error in wiki)

 

After installing Solr, wherever the installation sets up there should a folder similar to:

<your prefix>/solr/server/solr/configsets

If you look there, you'll probably see folders like '_default' and 'sample_techproducts_configs'.  I haven't played with the 'techproducts' sample.  Copy the '_default' folder, with all its contents, to a 'dovecot' folder.  In the new dovecot folder, replace the 'managed-schema' file with the file from the Dovecot Wiki

https://wiki.dovecot.org/Plugins/FTS/Solr?action=AttachFile&do=view&target=solr-7.x-schema.xml

after that, you should be able to run 'solr /opt/solr/bin/solr create -c dovecot' to create the instance.  If things still don't work let us know.

The schema is one I've tweaked and updated during my own migrations since Solr 3.3.  It's possible there's something else in my config that needs documenting - but having experienced Solr search against my mailstore I never want to be without it.

Daniel