I have a problem with incoming links, plugins and wordpress development log

I have installed and battled my way through to the stage where in the three boxes incoming links, plugins and wordpress development log in dashboard I have the following text/error message.

Fatal error: Class wp_feed_cache: Cannot inherit from undefined class simplepie_cache in /home/users/uks19066/html/flashboy76.com/blog/wp-includes/class-feed.php on line 6.

Now the witty answer I expect is that the simplepie_cashe is undefined, which is all very well but I am trying to use this as a tool and I am pretty much a complete novice.
Please don’t answer with ‘it tells you what the problem is’ because if I could understand that then I wouldn’t need to beg for some help.

I think the work you do is great and hope to take advantage of this blog software so I would be grateful for your help.