I have started using an older Mac with OS 10.11, however, though logged in with my icloud id, my files from the current Mac's desktop are not uploaded to the icloud. what shall I do

Note that even though the Desktop & Documents option isn't available with 10.11.6, you can sync files/folders with iCloud Drive by moving them to the iCloud Drive folder in Finder.

The Desktop and Documents option of iCloud Drive is a feature that was introduced with macOS 10.12.

Ijazk wrote:

Thank you, but my OS is 10.11.6

And that explains why iCloud Desktop & Documents are not syncing. That feature was not introduced until 10.12 and your OS is before that.