Originally Posted by Sykes

As others have said, the Google Flights -> United.com connection is a bit wonky so a lot of time these errors just happen randomly, but one common tip I have is that once you get to the payment screen don't change the country for your billing address. When using the connection, United.com will assume a point of sale of the origin of the flight (TLV in your case), and in some cases you may get favorable fares and/or inventory with that point of sale that would be lost if you change the country of your billing address. I personally haven't had any trouble with payment processing with the wrong billing address country. (This may not be applicable to OP, but it's a common issue I've run into.)