MacOS Mojave: Can not make changing background wallpaper

Opening "SystemPreferences", then "Desktop & Screen Saver", and then the "Desktop" tab:

I would like to add my own folder holding pictures to display as a background, and to change these pictures in certain intervals.

I am able to add (with the "+" button) on the left the directory which holds by picture, but the checkboxes "Change picture" and "Random order" on the right are greyed out. How can I enable this?

You need to select any one photo in the folder you made to be able to see the change picture button, checking which will enable random too.

Other words, shift focus from side-bar of folders to photos.

Once you have moved to photos pane, enabled change, enabled random, background pictures will automatically be picked from the parent directory.