plus/minus buttons aren’t a part of core WooCommerce. That’s coming from your theme, Flatsome. Though last time I used Flatsome their buttons work fine. I’m fairly confident this isn’t related to WooCommerce and would recommend a classic debug routine: start disabling plugins one at a time until it starts working. Normally, I’d say switch to a default theme, but those don’t have the plus minus buttons.
I just noticed, your site is missing a closing </html> . This suggests to me that you have a fatal error somewhere. This can prevent all your scripts from loading… which would explain why I don’t see the Flatsome plus/minus scripts in the developer console. You can check your WooCommerce > Status > Logs for fatal errors. Or try enabling WP_DEBUG.