Check the documentation here: https://docs.mendix.com/developerportal/operate/backups
It is usefull to download a backup to see how the tar file should be structured.
Regards,
Ronald
[EDIT]
To help you further I need to know your local postres version you are using and if you are trying to upload to V3 or V4 cloud.
And the structure should be db with db.backup containing the database and tree with all your subfolders 00, 01 etc. These directories should be zipped with a .tar extension and then again zipped with a gz extension.