Exception: variable does not exist in rstan:failed to create the sampler - sampling

Question

I`ve tried finding a similar error to mine on the site but I cannot find anything. I am not sure what am I doing wrong because I keep getting the below error message. I would greatly appreciate it if somebody could explain why do I keep getting an error.TIA
error message:
Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) :
Exception: variable does not exist; processing stage=data initialization; variable name=j; base type=int (in 'model32487fbf6c2c_production' at line 2)
failed to create the sampler; sampling not done
Rcode
library(rstan)
dat1 <- list(a1=0, b1= 1, a2 =0.1, b2 =0.5, j <-6, n <-c(3,7,4,8,5,9), x <-c(10,33,3,39,5,50))
fit1 <- stan(file = "production.stan", data = dat1, chains = 3, iter = 1000,)
print(fit1)
Stan code
data {
int < lower = 0 > j;
int x[j];
int n[j];
real a1;
real b1;
real a2;
real b2;
}
parameters {
real < lower = 0 > lambda[j];
real < lower = 0 > sigma;
real mu;
}
transformed parameters {
real lambdanew1 = lambda[1]. / n[1];
real lambdanew2 = lambda[2]. / n[2];
real lambdanew3 = lambda[3]. / n[3];
real lambdanew4 = lambda[4]. / n[4];
real lambdanew5 = lambda[5]. / n[5];
real lambdanew6 = lambda[6]. / n[6];
}
model {
target += poisson_lpmf(x[1] | lambda[1]);
target += poisson_lpmf(x[2] | lambda[2]);
target += poisson_lpmf(x[3] | lambda[3]);
target += poisson_lpmf(x[4] | lambda[4]);
target += poisson_lpmf(x[5] | lambda[5]);
target += poisson_lpmf(x[6] | lambda[6]);
target += lognormal_lpdf(lambdanew1 | mu, sigma);
target += lognormal_lpdf(lambdanew2 | mu, sigma);
target += lognormal_lpdf(lambdanew3 | mu, sigma);
target += lognormal_lpdf(lambdanew4 | mu, sigma);
target += lognormal_lpdf(lambdanew5 | mu, sigma);
target += lognormal_lpdf(lambdanew6 | mu, sigma);
target += uniform_lpdf(mu | a1, b1);
target += uniform_lpdf(sigma | a2, b2);
}