So - sometimes this is due to phantom availability as others have said - but having worked in digital analytics at another airline in a past life and tackling a similar issue I can recommend the following:
- Check that you have filled out all "required" fields on the payment page. Sometimes the booking engine will throw a generic error instead of a validation error
- Clear your browser's cookies and cache, quit the browser, and start over
- Try booking in a new browser or on a different channel such as mobile web or app
- If you have an ad blocker or similar privacy extensions installed on google chrome, try disabling those or doing the search/purchase process in an incognito window. Sometimes extensions get updated and wreak havoc for weeks-months.

As a general rule of thumb, if you get this error, take a look at your credit card's app and see if a pending charge has appeared. If it hasn't, this is indicative of some sort of validation error on United's site (not related to your credit card's ability to process the transaction).