Maybe it is because of the dimension selected there.
First, an image must be of banner size.
Check the steps below
1. Go to the page you want to add a banner
2. Click Upload
3. Select already uploaded image or upload a new image.
4. Before setting this image as a banner, on the right side at the bottom it has an option of “Attachments display settings”. Under that, You will see the option of size.
5. Select the full size from the drop-down.
6. Then set the image.
7. Publish the page. It will work.
If it is still showing the error send us the page URL we can check.