If this confusing WordPress error appears when you’re trying to upload the theme, it means the resources set of your server are too limited for your site. Usually, the maximum upload size is set to 5 MB. Unfortunately, the theme’s package is larger than that. This isn’t a bug but a matter of server settings and you should increase that limit. Otherwise, even uploading a larger image could cause the very same issue.
You can either reach out to your hosting company and ask them to increase it for your website, or do it yourself. In this case, we suggest you to add the following snippet via FTP at the .htaccess file:
php_value post_max_size 32M
php_value upload_max_filesize 32M
php_value memory_limit 256M
php_value max_execution_time 300
php_value max_input_time 300
php_value session.gc_maxlifetime 86400