Since red was the color associated with communists, why did the Nationalists/Fascists in the Spanish Civil War wear red shirts?

The red comes from the Carlist side of the Falange.
It was worn by Basque troops.
https://en.wikipedia.org/wiki/Carlist#Symbolism

The flag of the Falange movement was red and black.
https://en.wikipedia.org/wiki/Symbols_of_Francoism

Except that the Carlist symbol was a red beret, not a shirt, and the Nationalists/Fascists/Falangists wore blue shirts: "After the union with the Carlists, the original Falange uniform became important in identifying genuine Falangists.[17] The red beret had become the symbol of carlism by the 1860s.[21] The new party, commonly known as the Movimiento Nacional, was given a uniform with the Falangist blue shirt, the red Carlist beret and military belts." -from https://en.wikipedia.org/wiki/Symbols_of_Francoism

The same basic home uniform was used for the volunteer Blue Division that fought in the USSR for Hitler in World War II: "Because the soldiers could not use official Spanish army uniforms, they adopted a symbolic uniform comprising the red berets of the Carlists, khaki trousers used in the Spanish Legion, and the blue shirts of the Falangists - hence the nickname "Blue Division." This uniform was used only while on leave in Spain; in the field, soldiers wore the German Army (Wehrmacht Heer) field grey uniform with a shield on the upper right sleeve bearing the word "España" and the Spanish national colours." -from https://en.wikipedia.org/wiki/Blue_Division