Can I use WP RSS Aggregator on a WordPress.com-hosted site?
Unfortunately, WordPress.com does not allow our core plugin and add-ons to be used on any sites they host.
This is explained on their website here: https://en.support.wordpress.com/incompatible-plugins/
As they explain it:
We know it can be tempting to automatically import content from around the web using content scrapers. We’ve found that these sorts of plugins are not a good fit for sites hosted here at WordPress.com and ask that you refrain from using them as not only do they place unnecessary loads on our infrastructure, automated posting is often seen as spam and is generally unwanted.
Although our plugins can be used for many great purposes and we've had no such issues with any other top WordPress hosting companies, this is something out of our control. We have reached out to the WordPress.com team, however, they have no plans of changing their reasoning.