You can safely ignore this error message as it will not affect your ability to post the item in question. This issue will be corrected with out next site update, which will take place some time next week.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .