Private file system, The upload directory private://webform/XXXX/_sid_ for the file field could not be created or is not accessible
Your path must exist outside of your webroot directory and be an absolute path, for example: /var/www/private )

Also check PHP configuration


file_uploads = On must be set to "On"
upload_max_filesize = 24M can't be larger than post_max_size
max_input_time = 300 small values may cause timeouts for large file
uploads
memory_limit = 64M small values may cause out of memory errors for
large file uploads
max_execution_time = 180 small values may cause timeouts for large
file uploads
post_max_size = 24M limits the size of input submitted to the website
(including attached files)


Information is taken from here.