I had the same issue when trying to embed an image in my discussion post. my storage was too full on canvas. go to profile > files > my files. next select any documents or images you no longer need (surprisingly a lot of uploads will be there). Now select the three dots on the side of the selected file and click delete from the drop down menu. now refresh your page and look at the bottom left hand corner. this should have a bar indicating how much space you have left on canvas. If it is below 100% you will be able to successfully upload. hope this helps!