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Does this file already exist? If it does, you can try deleting existing files with wrong ownership/permissions (basically, the wp-content/uploads directory) and retry the restoration.

If the file does not exist check the available disk space. Do remember that cPanel does not always show up-to-date information, or even all of the relevant information. Best ask your host if you have exceeded any limits such as your assigned disk space quota, or your account's inode limit.

You have already determined that neither the backup nor the backup extraction script is a problem by extracting it on a known good server. So, we can rule out a broken backup as the issue.

The "not uploaded" message might be confusing you. It really means "cannot write into this file". We need to know why the file is not being written to give you appropriate guidance.

You might be running out of free space. You said you have already checked that, so I am not expanding further.

You might be running out of your inode limit, i.e. the maximum number of file you are allowed to create in your hosting account. You have to ask your host if this is the case. If this is the case, I cannot help you; it's a limit imposed by the hosting company you selected and you'd need to talk it through with them.

You might have a permissions issue which does not allow writing into the file or directory. That's why I asked you if there are existing files or folders. If this is the case, I already told you that you can try removing the existing files to allow the restoration to proceed.