On a usual Debian-based system, one would install this plugin through the package dovecot-solr and call it a day. But Plesk ships its own version of dovecot (plesk-dovecot) without Solr support, which conflicts with dovecot-solr.
The obvious solution to this is to purge Plesk from your system, but that might not be possible.
But, you can of course build the fts_solr plugin manually.
You shouldn’t do this, unless you have to. Stuff will break.