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('/Shared Documents')

Reference:

Working with folders and files with REST.

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('/Shared Documents')

Reference:

Working with folders and files with REST.

Also, you need to create the Site level app and add authorization to Postman. Check below article for this:

Access SharePoint Online using Postman.

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('/Shared Documents')

Reference:

Working with folders and files with REST.