Thank you for reaching out and bringing this to our attention!
I understand your store is experiencing an issue when trying to add items to cart. It's possible that this type of issue is due to a custom script or a third party app, as they are the more common causes when they aren't installed correctly. Were there any custom coding or apps installed recently that may have lead to this issue?
If you're not sure, then one method to check what might be causing the issue is by installing a new free theme. This will allow you to see if the issue persists and determine whether its caused by an app or custom coding. Since the new theme doesn't have any custom coding and if the issue persists, then it's likely caused by an app.
If you're unable to identify the issue or the issue persists with a new theme, then it looks like we'll need to access your account in order to resolve the issue. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visit the Shopify Help Center and log in to your account to create a support request.
I'd love to know if our Support team can help you identify and resolve the issue. Keep me posted!