Spain has cracked down on transit. If the trip was today, you would have likely been out of luck and denied travel unless you could demonstrate proof of Spanish citizenship, Spanish residency or had some other exemption to the travel ban Spain has set up in response to the concerns about a more transmissible UK variant of the virus that causes Covid-19.

The FT Hyatt forum moderator Markie had just such an incident today while trying to fly from the UK to transit Spain to go beyond.

Call up the ticketing airline/agent and have them book you on a route that involves transit elsewhere -- unless you have a Spanish passport, Spanish residency or otherwise are in the minority with some other exemption available to you to transit Spain at this time. Even then, if you are intent upon traveling, I would still suggest trying to get the ticketing airline/agent to route you to your destination via somewhere in the Americas that is popular with American tourists and open to visiting tourists from the US at this time instead of counting on a transit in the EU/Schengen area.