You probably don't have an Asset source defined yet. The button won't show up if there are no Asset sources, regardless of what your Redactor config specifies.

You probably don't have an Asset Volume defined yet. The button won't show up if there are no Asset Volumes, regardless of what your Redactor config specifies.

To add an Asset Volume, visit the Settings section of the admin, click "Assets," and then "Volumes." By default, only the "Local" Volume type is available, but you can install additional Volume types through the Plugin Store.