Firstly, please keep in mind the technical restrictions on uploading comic pages:
Please ensure your file meets these criteria. If you are trying to upload a .bmp or a file larger than 2 Mb, it will be rejected.
If you get an error message, take note of it. It might tell you what is wrong. If it's a "code" error (like 403 Forbidden, 502 Bad Gateway, 504 Gateway Timeout, etc.) then that is likely a problem on the server/programming end and may need to be reported.
While The Duck is still ironing out some bugs, it may be worthwhile to try uploading the page using a different browser. Although that's not a long-term solution, it might get the job done, and if you report the problem along with browser information it can help the programmers identify and fix the problem.
You can ask an admin or moderator to help you. They will most likely ask you to tell them what you tried, send them the page you're trying to upload, and ask to be made an assistant (temporarily) on your comic. That way they can try uploading the page themselves to see if that helps figure out what the problem might be.