Hi @AlejandraHolden,
I would definitely suggest that you contact Canvas support and let them know the message you're receiving so they can investigate the cause of that error. If the system allows you to enter a grade, it *should* save the grade as well. In every situation I can think of where you might not be able to grade, Canvas would gray out the grade/points box and not let you type anything. One workaround/suggestion I could make is to try using a different web browser if you can, just in case there is a browser issue or some kind of extension blocking proper communication with Canvas.
Hope this helps a bit! Let us know if you're able to get this working.
-Chris