Yes you can. As a general rule, it’ll work anywhere where MasterCard cards are accepted. The known issues for Monzo (which are mainly due to the fact that it’s a prepaid card) are listed here.
You’ll find more tips on using Monzo in Spain here -
If you have any additional questions, please ask them there to make it easy for other users to find the advice.
The complete list of dedicated pages for advice on using Monzo in countries around the world is here -