A US 120v power adaptor will not work on Spanish/European 240v mains power. You need to buy a 240v AC -> 24v AC power adaptor.
If it helps, I installed a Rachio 2 in my house in Spain about 3 years ago and I have had no problems at all (with the electrical supply, anyway!).
Mine is fitted outside so in the Rachio 2 outside enclosure which is big enough to fit a UK power supply plugged into a trailing socket behind the Rachio 2. The trailing socket has a EU plug on the other end that plugs into the outside power socket.
Here are the links to the power supply I have used (twice, one in UK as well) and also the trailing socket .
Alternatively you could search for a suitable power supply with an EU plug, but would you want to plug that directly into an outside socket?