If the Active Sequence upload fails, it is good to keep an eye on Adobe's Media Encoder (AME). Sometimes AME gives you a good hint of what the issue could be. But in general, if it fails to render the Sequence, the upload will logically fail, and the Panel shows a red circle.
When you see a yellow warning sign next to "Failed," you can click it in Media Encoder, and a text editor launches with a long text document. Typically the last line of this long log will tell what the issue is. Feel free to send the entire log to Support to troubleshoot the issue further.
Incrementing the File Name
If you repeatedly upload the Sequence without changing the scratch disk path and the Sequence's name, AME is likely to fail as it does not overwrite existing files. The workaround is to enable Preferences ... > General > "Increment output file name if a file with the same name exists." AME will automatically append an incrementing number to your render output file names.
Testing
As we start to discover and figure out solutions for failed uploads, it is good first to check if you can queue/ render the sequence directly in AME without the Frame.io Panel. We had several occasions where queueing the sequence did not work, which could only be solved by re-installing Premiere Pro.
Permissions
Another reason for users reporting failed uploads was that AME could not write files to a certain location. To test if it is a permission issue, try to set the render output location to Custom Folder... and select your Desktop. If it renders out fine and the upload starts, you know that the permissions must be adjusted on your former output location.
Folder Doesn't Exist
When the log text says the following:
An error was encountered while writing the output file. The output destination could not be found. Check that the output directory still exists. If the output directory is on a network or external device, check that the connection is active.
Be sure to set the render location again. If you're trying to render to a shared network drive, Media Encoder may not allow this, and it would be best to set the location to a local drive on your computer to test.