Cannot Upload Theme

Preface to what follows: I’m new to this and still learning.

I purchased a theme, downloaded the .zip file, and have attempted numerous times to upload the theme. I keep getting the error message:

Are you sure you want to do this?

Please try again.
Forbidden

You don’t have permission to access /wp-admin/update.php on this server.

The seller of the theme seems to have disappeared, his website is gone and emails are returned. I searched the forums for a similar issue, but did not find anything. Please tell me this is a simple fix.

Many thanks!