This error is misleading and we are working with Roblox to improve the message 😩. It simply means you are logged into a different Roblox account on the Roblox website compared to the one logged into Roblox Studio.
Once signed into the same account on Roblox Studio and their website, you can install the plugin.
Please check out our article below that goes through the process of setting up Roblox Studio and our plugin 👍