iCloud not completing upload of my 74G file, message says "try later"

The file size exceeds the limit.

What types of files can I store in iCloud Drive?

You can store any type of file in iCloud Drive, as long as it's 50GB or less in size and you don't exceed your iCloud storage limit. That means you can keep all of your work documents, school projects, presentations, and more up to date across all of your devices. Learn more about managing your iCloud storage.

iCloud Drive FAQ

See this article Add your Desktop and Documents files to iCloud Drive - Apple Support

Your files upload to iCloud each time your device connects to the Internet. Depending on your Internet speed, the time it takes for you to see your files on your other devices might vary.

also it depends upon your cloud storage are you using free 5GB iCloud storage , or exceeded the limits you can read some article for it Manage your iCloud storage - Apple Support

How to free up storage space on your Mac - Apple Support

Buy more iCloud storage - Apple Support

Try to instal latest version like Mac OS Mojave 10.14 ( make sure the Mac has compatibility with latest versions ) see this article Upgrade to macOS Mojave - Official Apple Support

Thanks ! for the information and appreciate it .

You are welcome.