It is very strange. This error can occur only if something wrong with the plugin settings, because this part of the plugin read the saved plugin settings, and found that it does not contain the “wri_used_posttypes” setting (the checkboxes that I mentioned above).
Please attache a screenshot about the General tab, the Post tab and Product tab of WP Related Items settings page, maybe this help to find the reason of it.
You can also try to reinstall (inactivate the plugin, then overwrite the files and activate it again), maybe something went wrong during installation.
Then please save again the plugin settings, so change something and save it.