Some networks (Instagram, VK, etc..) require the actual image file to be uploaded. If plugin can’t read the local file it must grab it and save it in the temporary folder before uploading.
Plugin asks the server system to get it the writable temporary file by calling standard PHP function “tmpfile()” (https://php.net/manual/en/function.tmpfile.php). This function suppose to make writable file in your system defined temporary folder.
If tmpfile returns wrong temporary folder or file is not writable, it means your server or PHP itself is misconfigured and your hosting company must fix that. Please contact your server admin or hosting provider and ask them why standard PHP function “tmpfile()” is not working on your server.