Just toggle your indexer mode from
realtime and then back to
schedule. You can use the Magento Admin (System->Index Management) or the command line tool thus:
bin/magento indexer:set-mode realtime
bin/magento indexer:set-mode schedule
As described in the Magento2 documentation.
The error is due to some tables eg
catalog_product_entity_int having triggers on them so that the scheduled indexer knows what to index. The module CatalogStaging (or one of its dependencies) changes a column name in the table from
row_id but the trigger does not get updated so it still refers to
NEW.entity_id when the
entity_id column no longer exists.
The triggers are created when the index mode is set to
scheduled so this situation may affect any DB of a Magento install that has been set to index on schedule before upgrading to 2.1.
The fix is simple but not obvious : toggle the indexer to
realtime (update on save) and then put it back to
schedule. This clears the triggers and then recreates them with the correct reference to the updated column name:
NEW.row_id in the trigger as now required by the Magento_CatalogStaging module