I see you are experiencing a JS error based on your developer console screenshot. If you’re using a cache plugin, that could explain that issue. PayPal not being defined indicates the dependency script https://www.paypal.com/sdk/js is either not loading or is loading out of sync with it’s dependent scripts.
Dear author: I did not use any caching plugins, and this issue occurred after multiple tests and checkout. It may be an accidental bug, thank you very much for your reply