In Your URL you have added Shared%20Documents
after site URL, which is not required.
Try using below URL:
https://learningsfdc.sharepoint.com/sites/SidFirstSite/_api/web/getfolderbyserverrelativeurl('/sites/SidFirstSite/Shared%20Documents')
In Your URL you have added Shared%20Documents
after site URL, which is not required.
Try using below URL:
https://learningsfdc.sharepoint.com/sites/SidFirstSite/_api/web/getfolderbyserverrelativeurl('/sites/SidFirstSite/Shared%20Documents')