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](https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/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](https://www.ktskumar.com/2017/01/access-sharepoint-online-using-postman/).