You seem to be missing the parameters that are to be inserted.
See the examples at http://justinpdavis.blogspot.com/2010/04/logging-to-database-with-nlog.html
The nLog web page doesn’t make it very clear that these are required, but if you squint your eyes and read https://github.com/nlog/NLog/wiki/Database-target you should find that they are required.