Updating plugins

Just trying to update Subscribe by Email plugin and received the following error message:

Downloading update from https://premium.wpmudev.org/wdp-un.php?action=download&key=xxxxxxxxxxxxxxxxxxxxxxx…

Unpacking the update…

Could not create directory. /home/realesta/public_html/wp-content/upgrade/wdp-un.tmp

Can you please let me know why.