Button for imagemanager plugin not appearing in toolbar

[brimby]

Description

Enabling the imagemanager plugin does not cause the button to appear in the toolbar.

Steps to reproduce

Here is my Standard.json config file I'm using. I've tried cutting out all the other plugins and buttons (including "image") and they all disappear but imagemanager never appears.
{ "buttons": ["html","formatting", "bold", "italic", "unorderedlist", "orderedlist", "link", "image", "file", "video"], "plugins": ["fullscreen", "video", "email", "table", "imagemanager"] }

Additional info

• Craft version: 3.8.17
• PHP version: 7.2.34
• Database driver & version: MySQL 5.7.34
• Plugins & versions: Redactor 2.10.12