How to set up Supabase as a Laravel file system?

I want to use Supabase as a filesystem so I can easily upload files to Supabase. I used this package (github.com/quix-labs/laravel-supabase-flysystem) to set it up and added the supabase block to the disks array in config/filesystems.php. However, I can't upload anything to Firebase - the method that actually uploads the file returns this error message: "Unable to write file at location: uploads/RQhhQkjeHerFo5f1OxUgU0LMa40Kulif52tUKzzM.png."

I'm not sure I've set everything up correctly. I have this config:

'supabase' => [
            'driver' => 'supabase',
            'key'    => env('SUPABASE_SECRET_ACCESS_KEY'), // Use a privileged key; read-only does not work
            'bucket' => env('SUPABASE_STORAGE_BUCKET'),
            'endpoint' => env('SUPABASE_STORAGE_ENDPOINT'),

            'url'      => null, // <- Automatically generated; change here if you are using a proxy

            'public'                      => true,  // Default to true
            'defaultUrlGeneration'        => null, // 'signed' | 'public' <- default depends on public

            'defaultUrlGenerationOptions' => [
                'download'  => false,
                'transform' => [],
            ],

            'signedUrlExpires' => 60 * 60 * 24, // 1 day <- default to 1 hour (3600)
        ],

I'm guessing the bucket is simply the name of the bucket (e.g. myapp-file-uploads) and the endpoint is the one specified in the storage settings.

The key is the bit I don't understand. I don't know what "Use a privileged key; read-only does not work" means. I've tried the S3 key in the storage settings and the Project API keys in the API settings.

Thanks for any help anyone can provide.