Cannot use libraries in sub-theme
Simply add your library here in your's my libraries.yml file

 bootstrap-scripts:
 js:
bootstrap/js/affix.js: {}
bootstrap/js/alert.js: {}
bootstrap/js/button.js: {}
bootstrap/js/carousel.js: {}
bootstrap/js/collapse.js: {}
bootstrap/js/dropdown.js: {}
bootstrap/js/modal.js: {}
bootstrap/js/tooltip.js: {}
bootstrap/js/popover.js: {}
bootstrap/js/scrollspy.js: {}
bootstrap/js/tab.js: {}
bootstrap/js/transition.js: {}
bootstrap/js/transition.js: {}

Your-sub-theme/js/jquery.countdown.min.js: {}

Your-sub-theme/js/auction-timer.js: {}


**No need to define it separately also place your js file at correct folder path should be eg:Your-sub-theme/js/jquery.countdown.min.js then clear cache **.