Can’t upload CSV file to plugin directory using custom upload form in admin panel

I don’t know what the reason you upload a file to the plugin directory. The standard way to upload a file is to the WordPress uploads directory.

In your code, I find this one is wrong

$uploadPath = $currentDir . $uploadDirectory . basename($fileName); 

See, you merge two path together. Try

$uploadPath = $uploadDirectory . basename($fileName); 

But again, it is not the correct way, you should upload files to wp_upload_dir() instead.