You need to check the google analytic and Facebook ad set up to find out the exact reason. but let me just clear few things due to which it normally happens.
When you watch Google analytic closely notice if you see Multiple "visitors" from the same IP at the same time? and then instantly see add to cart. You can block that id to prevent fraud or spam traffic. You can also use captcha to avoid bots.
Facebook ads can be a great way to increase traffic numbers for your website. However, if you have yet to see an adequate return on investment, it may just be that you’re making some common mistakes leading up to getting your custom audience to your landing page.
I would suggest to check your check out process. Is there any hurdles? I would recommend to make the checkout process as simple as possible for your shoppers.
Also work on to build customer trust, add some trust signals. Trust Signals build customers trust and help users to feel more secure in their buying decision.
Send abandoned cart emails: If you have lots of traffic and little sales due to abandoned carts so you can get lost revenue using abandonment email campaigns. There are many Automated tools for abandoned carts. They can help you increase sales and re-engage visitors to your site who have left. You can also send promotional emails to your subscriber's list.
I hope you like my feedback. Thank you!