are you seeing the error only in the log files?
Is the site displaying correctly other than the error in the log file?
This error would happen if you tried to access the webassist/themes/SilkPlantsCanada2/plugins/plugins.php directly, which is not intended.
That file is used to add items like the popular box and the news pod to the index page but not on others. It is also used to load features like the footer, menu and search box on the other pages of the site.
if these features are functioning properly in your site, the error can be ignored in the error log.