on a shared server hosting you may need to provide the complete relative path to your upload folder. Also make sure you have all permissions to write a file to that directory!
you can use on a localhost environment
$config['upload_path'] = './assets/upload';
but on a shared hosting, you'll need to get more specific, normally something like
$config['upload_path'] = '/home/yourserver/public_html/assets/upload';
You can find this upload_path, e.g. in your accounts cPanel main page on the left column or might want to call your provider's helpdesk for more info on the correct path.
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