TLDR; There could be many reasons for this. First make sure you added the Checkout X script to your theme.liquid file. If the script is in place, you'll need to do some HTML diagnostics ( it's a bit advanced 🙀) or ping our support.
Before continuing, make sure you added the Checkout X script to your theme.
Let's diagnose the button:
When using Chrome, Right-click the Checkout button and select "Inspect" from the dropdown menu. You should see something like this:
Depending on what you see there, you should be able to figure out what's wrong.
There is a "data-ocu-checkout" attribute on the checkout button
This problem occurs when One Click Upsell has their script on your shop. Usually the user already uninstalled the OCU app, however, the code remains...
To fix this, go to the your theme's code and delete the file
upsellapp-theme.liquid ; This will fix the issue