Hi, try to enable debug mode in Joomla! if it says something more. Unfortunately, Phoca Download (even any Phoca extension) does not install itselves, it is installed by Joomla! extension manager, so the solution needs to be found there.
Try to enable debug mode an install the file, try to see php errorLog on your server, if there is no error message after installing the file.
Copy file failed mostly means that Joomla! extension manager gets info from the server that it just cannot copy the file to the server and mostly this is caused by permissions or ownership

But what exactly can be wrong there, really hard to say, try to see the debug and errorLog
Jan