Unfortunately, because you’re on Windows it’s probably because of this current mess we’re dealing with:
To run Stan from R without the hassles of RStan on Windows I recommend our new CmdStanR interface. Here are some nice instructions from @Max_Mantei:
Your data block compiles fine for me with the latest CmdStanR:
library(cmdstanr)
file <- write_stan_tempfile(
"
data{
int LENGTH;
matrix[LENGTH,2] RT;
int NUM_CHOICES;
}
"
)
# compiles with no error
mod <- cmdstan_model(file)
For more info on CmdStanR we have some tutorials at Articles • cmdstanr.