Cubarsi, who was one of Barcelona's best players in the second half of last season, started Wednesday's international friendly with Andorra, playing the first half of the contest.
The teenager had widely been expected to be named in the final squad for Euro 2024, and there had also been claims that he could be in the starting XI due to his performances at club level after breaking into the side.
However, Cubarsi has been dropped from the final squad, while there is also no spot for either Atletico Madrid midfielder Marcos Llorente or Girona midfielder Aleix Garcia.
Both Llorente and Garcia started against Andorra, with the latter playing 67 minutes of the contest, but head coach Luis de la Fuente has dropped the pair alongside Cubarsi.
Cubarsi, Llorente, Garcia drop out of Spain squad
Aymeric Laporte, Nacho, Robin Le Normand and Dani Vivian have been selected as the four centre-backs, with the latter's strong form for Athletic Bilbao seeing him earn a spot.
Laporte and Le Normand were guaranteed to be in the initial selection, while Nacho enjoyed an impressive 2023-24 campaign for Real Madrid, so his spot in the squad did not come as a surprise.
Spain are also blessed with a host of talent in central midfield, with Rodri, Martin Zubimendi, Fabian Ruiz, Mikel Merino, Pedri, Alex Baena and Fermin Lopez making the cut.
Fermin made his debut for Spain against Andorra, while the same can also be said for Ayoze Perez, with the Real Betis attacker capping his first appearance with a goal.
Spain are searching for a fourth European Championship triumph
Spain have won the European Championship on three previous occasions, with their first success coming in 1964, while they also famously lifted the trophy in 2008 and 2012.
La Roja made the semi-finals of Euro 2020, meanwhile, but they disappointed at their last major tournament, being eliminated in the last-16 stage of the World Cup.
France and England are seen as the favourites to win this summer's competition, but Spain are arguably in the group behind with Portugal and Germany when it comes to challengers for the trophy.
The quality in the squad means that they must be taken extremely seriously, and it would be a surprise if they did not progress from Group B, which also includes Croatia, Italy and Albania.
De la Fuente's side will play their final pre-tournament warm-up fixture against the Republic of Ireland on Saturday before beginning their Euro 2024 challenge against Croatia on June 15.
Spain's Euro 2024 squad in full
Goalkeepers: Alex Remiro (Real Sociedad), David Raya (Arsenal), Unai Simon (Athletic Bilbao)
Defenders: Aymeric Laporte (Al-Nassr), Robin Le Normand (Real Sociedad), Alex Grimaldo (Bayer Leverkusen), Dani Carvajal (Real Madrid), Dani Vivian (Athletic Bilbao), Jesus Navas (Sevilla), Nacho (Real Madrid), Marc Cucarella (Chelsea)
Midfielders: Mikel Merino (Real Sociedad), Fabian Ruiz (Paris Saint-Germain), Alex Baena (Villarreal), Martin Zubimendi (Real Sociedad), Rodri (Manchester City), Pedri (Barcelona), Fermin Lopez (Barcelona)
Forwards: Alvaro Morata (Atletico Madrid), Dani Olmo (RB Leipzig), Joselu (Real Madrid), Lamine Yamal (Barcelona), Mikel Oyarzabal (Real Sociedad), Ayoze Perez (Real Betis), Ferran Torres (Barcelona), Nico Williams (Athletic Bilbao)