I am able to copy a photo onto the post but when I click post or reply, it won't post.
Unfortunately you can't copy/paste images into discussions in Canvas. As a student, you need to first upload the image to your files area, and then you can embed it in your discussion.
These guides might help.
How do I upload a file to my user or group files?
How do I embed images from Canvas into the Rich Content Editor as a student?
I also have a video showing the process if that helps.
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