Hello, thank you for the answer. I went back to use the original uno pinout, so now it's:
const byte latchPin=8;
const byte clockPin=12;
const byte data_R1=10;
const byte data_R2=11;
const byte en_74138=2;
const byte la_74138=3;
const byte lb_74138=4;
const byte lc_74138=5;
const byte ld_74138=6;
I saw online that the mega has pin 10, 11, 12, 13 on port b, so the pinout issue should be fixed, right?
But I don't understand how to change the "-8" part you mentioned since it's not compatible with arduino mega