Are you using Internet Explorer? If so, you may have a plug-in or malware that is preventing you from being able to paste into the Post Place.
My suggestion is to download the Chrome browser. It is way more secure than Internet Explorer and is way faster as well because it was created by Google. It is the only browser that we will let our employees use because it is so much better than Internet Explorer.
Here is a link to download it:
If you download it and you have no problems pasting in it then you definitely have an issue with your Internet Explorer.