The SDK may give a variety of errors depending on how your upload failed. Here's some steps to try.
First, ensure your SDk is up to date.
Updating your SDK with a fresh install is important. It solves many issues and a clean install is important between versions.
Here's how you do a proper SDK upgrade!
1. Please ensure you're using Unity 5.6.3p1. Download the latest version here: https://vrchat.com/download/sdk
2. Close Unity.
3. Find your Project folder with Windows Explorer, and find the Assets folder.
4. Delete the VRCSDK folder, as well as the VRCSDK.meta file in the Assets folder.
5. Go into Plugins, and delete the VRCSDK folder, as well as the VRCSDK.meta file in the Plugins folder.
6. Relaunch Unity, and then re-import the freshly-downloaded SDK. Do so by going to Assets, then Import Package, then Custom Package.
7. Select the new SDK (you probably saved it in Downloads)
8. Wait a moment, and click "Import" in the bottom left. If any files in the list are greyed out/unchecked, and you can't check them, you didn't delete everything you should have! Start over.
9. Wait a minute as the SDK finishes importing.
You can also try creating a new project, import the VRCSDK, and then import just the things you need to get your upload working.
It may also help to click "Detatch" on the Pipeline Manager in your avatar/world descriptor before trying another upload.
If you still have issues, find your Unity editor log and attach it to an email to firstname.lastname@example.org. You can find your Editor.log file at this location:
You can also find the file by pasting this into the address bar of a File Explorer window:
Please attach the Editor.log file to your email. Do not copy and paste it into the email!