I did not find the above remedies worked. I found another simple work around:
Add the directory var with permissions 777 to wp-content/plugins.
The command that is failing is a mkdir. Is NOT used if the var directory already exists. You may want to delete it for security reasons when you are finished all adds or uploads.
The reason fro the failure of the mkdir is not clear without reading more of the code. I am a newbie here and have not had the time yet.
