Welcome. This community is dedicated to Shopify Partners that provide apps, not for end support for themes/apps themselves.
That said, I advise you open your browser’s Console before clicking the Checkout button in your store.
There might be a clue to the origin of the issue, if there’s an error happening when you click checkout, it might appear in the console and you may be able to track it to a specific app or theme customization.
Here’s more specifics about opening the console in Chrome: Console overview | Chrome DevTools | Chrome for Developers