On the checkout page, the card number, expiration, and CVC fields are visible but do not accept any typing. Two things can cause this, and both clear without any change to your site.
Connection hiccup during loading. A brief connection hiccup while the checkout page is loading can produce a page where the payment boxes are visible but the secure card module behind them never finishes loading. Ask the guest to fully reload the page, or to start the booking again from the property page. A fresh checkout load normally resolves it.
Browser privacy tools. Privacy tools in the guest’s own browser, such as ad blockers, strict tracking protection, or privacy-focused browsers, can block the secure payment module from loading at all. If reloading does not help, ask the guest to try a different browser or a private or incognito window, or to temporarily disable content blocking for your site.
If the guest prefers not to retry. You can create the reservation manually in your PMS and send the guest a payment link.