Hi Wendy,
From the log, the error you are receiving is:
PCLZIP_ERR_READ_OPEN_FAIL (-2) : Unable to open temporary file 'pclzip-56a738ad925f2.tmp' in binary write mode
This suggests that PclZip doesn’t have write access to where it puts its temporary files. By default this is the root directory of your WordPress install. I would recommend asking your hosts to give WordPress and UpdraftPlus write access for the site’s root directory.
Alternatively you can change where it writes temporary files by adding the following line to your wp-config.php file:
define('PCLZIP_TEMPORARY_DIR', ABSPATH.'wp-content/uploads');
I would also recommend checking that your hosting has enough space to create the backup, as this error can also be caused by a full hosting account.
Best Wishes,
David