One common reason for not being able to send screenshots to chat is not having permission to write to the directory foldit saves screenshots to on your hard drive. When sending a screenshot to chat, a copy is saved to the directory listed in the options.txt file under "gui/image_dir". If you lack write permission to that directory, the save will fail, and the send to chat will also fail. Not sure if this is the case for you, but it's worth checking.