This is actually an ongoing Facebook bug.
Common Error Message
- Please reduce the amount of data you’re asking for, then retry your request
Again, this is a known Facebook API bug that affects many parts of their API (see the bug reports below). Facebook has said that the cause is their servers running out of memory. Most reports indicate that the problem is caused when including Page mentions in your post.
We've noticed that sometimes the post is published successfully to your page, even though Post Planner shows the post as failed. You should check your page to see if the post was published or not, then take action accordingly.
If retrying the post continues to fail, the Facebook API may be unable to publish the post with the page mention included. In that case, you can try posting natively on Facebook, or scheduling the post and adding the Page mention by hand afterwards.