You can’t add just any folder to the Start menu. Some common folders are included in a list from which you may choose. The list, which includes locations like Documents, Pictures, and Music, allows you to toggle on or off their display in the Start menu.

To personalize the Start menu to include your important locations:
  • Click or tap the Start button and choose Settings.
         
  • In Settings, scroll down the list and click or tap Personalization.
         
  • In the left navigation of the Personalization settings, click or tap Start. The Start settings will display to the right. Scroll down to the bottom of the Start settings, and click or tap Choose which folders appear on Start.
         
  • A list of the individual folders that can be added to the Start menu will display.
         
  • Turned on by default are choices like File Explorer and Settings. Toggle the switch from Off to On for the folders you want to add to your Start menu. In this example, we chose to add Personal Folder. Once you have made your selections, close the Settings window to save the changes. The next time you click the Start button, the item(s) you selected will display.
         
  • While you are there, you have the option to pin the folder to the Start screen. Just right-click on the folder and choose Pin to Start.
         
  • A new tile will display in the Start screen, and remain there until you unpin it.
         

Source: https://techhelpkb.com/how-to-add-folders-to-the-windows-10-start-menu/