I haven't worked with sqllite plugin before but my guess is that Logstash has to have permissions to write to the sqllite database in order to create the since_table.
The source for this is here:
https://github.com/elastic/logstash-contrib/blob/master/lib/logstash/inputs/sqlite.rb
Check your file permissions and maybe allow the logstash user to update the sqllite db file.