Dimension found as (19,1) instead of 19

I am trying to run Generalized Fiducial Inference and have the following code. When I run this, I get the error that the dims declared are 19, but the dims found are (19,19). I have tried converting to an array, but haven’t had luck. Any ideas on how to fix?

library(rstan)
x<-list(1,2,3,2,7,2,4,8,24,78,6,24,21,2,3,7,3,58,246)
ds <- list(N = length(x), y = x)

gfi_model = "
data {
int<lower=0> N; // number of observations
int<lower=0> y[N]; // outcome variable

}
parameters {
real mu;
}
model {
mu ~ normal(0,10); // our prior for mu

}
generated quantities {
real effect;
effect = mu+mu^(3/2)*(N-mu/(mu));
}
"
fit <- stan(model_code = gfi_model, data = ds, iter = 1000, chains = 4)

Why not have x be a vector, i.e.,

x <- c(1,2,3,2,7,2,4,8,24,78,6,24,21,2,3,7,3,58,246)