It sounds like you are uploading something that is not an XForm to Aggregate? Aggregate reached end-of-life status on May 2021 and is no longer being updated. Please use ODK Central instead.
If perhaps you are using Briefcase, then your post does not contain enough information anyone to help.
- What version are you using?
- What steps can we take to reproduce the problem?
- What you have you tried to fix the problem?
- Anything else we should know or have? If you have a test form or screenshots or logs, please attach.