wp-o-matic error on campaign set-up

I installed wp-o-matic on my blog and went to set up my first campaign.

I got the following error message when attempting to add a new category in the campaign set up:

Call to undefined method SimplePie::set_item_limit() in /home/content/b/a/z/baza1955/html/wp-content/plugins/wp-o-matic/wpomatic.php on line 805

Can anyone help please?

Thank you,

Barry