Unable to install plugins on wordpress

Okay, so I’m using wordpress.com rather than my own independant hosting. I bought a domain name and I’ve been successfully posting, but I can’t install ANY plugins whatsoever. Every time I try and go to the plugins tab, it comes up with a bunch of empty slots. There are three tabs; All, Active & Inactive, and there is also a search tool.
HOWEVER, when I click on any of these it comes up with a dialog saying

“Want to add a store to your site?
Support for Shopify, Ecwid, and Gumroad is now available for WordPress.com Business.
Upgrade now”

I thought you could use plugins without upgrading to the business plan? I may upgrade at some point in the future but it’s not financially viable or even worth it at this stage, but there are several plugins that I’d like to install.

I have the plugins downloaded from their actual websites, but I’m pretty sure that’s for if you have your own server instance of wordpress, and you should be able to natively search with wordpress.com.

I’ve tried changing to the twenty eleven theme but it hasn’t helped.
Any advice would be greatly appreciated