Conditioning a parameter over time in SIR models

Modeling
1

Hi, I am triying to run this code in stan


smodel = "functions {
  real[] sir(real t, real[] y, real[] theta, 
             real[] x_r, int[] x_i) {
             
    real S = y[1];
    real E = y[2];
    real L = y[3];
    real L1 = y[4];
    real D = y[5];
    real C2 = y[6];
    real D2 = y[7];
    real N = x_i[1];

        
        
        // fixed parameters
        real M = 4590490.56;
        real mu = 1/(75.50);
        real alphae = 0.00002;
        real sigma1 = 0.01;
                real epsilon = 1.02;
        real rc = 0.00001;
        real thetaest= 0.75;
         real phi1= 0.05;
         real pc;
           if(t<18){
        pc = 0;
        }else{
        pc = 0.65;
        }
        
        // parameters to be estimated
       real nu =theta[1];
        real tau = theta[2];
        real sigma2 = theta[3];
          real omega = theta[4];   
    real d1t= theta[5];
    real betat= theta[6];
    real lambdat = (betat*D)/N;
        
        
    real dS_dt = M + alphae*E + sigma1*L + pc*sigma2*L1 + (thetaest*tau)*D  - (mu + lambdat)*S;
    real dE_dt = lambdat*(S + epsilon*L) - (mu+ nu + alphae)*E;
    real dL_dt = (1- phi1)*nu*E +(1 - thetaest)*tau*D+(1-pc)*sigma2*L1 - (mu + sigma1 + rc + omega + epsilon*lambdat)*L;
    real dL1_dt = rc*L - (mu +  sigma2)*L1;
    real dD_dt = phi1*nu*E + omega*L - (mu + d1t + tau)*D;
    real dC2_dt = phi1*nu*E + omega*L;
    real dD2_dt = d1t*D;
        
    return {dS_dt, dE_dt,dL_dt,dL1_dt, dD_dt,dC2_dt, dD2_dt};
  }
}

data {
  int<lower=1> n_days;
  real y0[7];
  int to;
  real t0;
  real ts[n_days];
  int N;
  int cases1[to];
  int cases2[to];
}
transformed data {
  real x_r[0];
  int x_i[1] = { N };
}
parameters {
// parameters to be estimated
        
        real <lower =0> tau;
        real <lower =0> sigma2;
        real <lower =0> omega;
        real <lower =0> nu;
        real <lower =0> d1t;
        real <lower =0> betat;
     
   
        
        real<lower=0> phi_inv;
}
transformed parameters{
real theta[6];
real y[n_days, 7];
real incidence[n_days];
real deaths[n_days];
  real phi = 1. / phi_inv;
{
  theta[1] = nu;
  theta[2] = tau;
  theta[3] = sigma2;
  theta[4] = omega;
  theta[5] = d1t;
   theta[6] = betat;
 
y  = integrate_ode_rk45(sir, y0, t0, ts,theta, x_r, x_i);
incidence[1] = 92670;
deaths[1] = 5879;
for (i in 2:n_days){
    incidence[i] = y[i, 6] - y[i-1, 6];
    deaths[i] = y[i, 7] - y[i-1, 7];
  }
}
}
model {
nu ~ normal (3,1) T[0,]; 
tau ~normal (0.80,0.15) T[0,];
sigma2 ~normal (1.2,0.10) T[0,];
omega ~normal (0.005,0.0015) T[0,];
d1t~normal (0.045,0.01) T[0,];
betat~normal (3.2,2) T[0,10];

phi_inv ~ exponential(5);
cases1 ~ neg_binomial_2(incidence[1:to], phi); 
cases2 ~ neg_binomial_2(deaths[1:to], phi); 

} 

generated quantities {
  real pred_cases1[n_days];
  real pred_cases2[n_days];
 // pred_cases1 = neg_binomial_2_rng(incidence, phi);
 pred_cases1 = incidence;
  //pred_cases2 = neg_binomial_2_rng(deaths, phi);
   pred_cases2 = deaths;
}
"

The model runs well when I do not condition the parameter pc over time, but when I include this part (as the code above):

  if(t<18){
        pc = 0;
        }else{
        pc = 0.65;
        }

the code throws me an error like this:

SYNTAX ERROR, MESSAGE(S) FROM PARSER:
Variable "real" does not exist.
  31:         
    32:         // parameters to be estimated
    33:        real nu =theta[1];
                  ^
    34:         real tau = theta[2];
  -------------------------------------------------

it does not look like a problem with the ordinary differential equations solver in STAN, Would you help me please?

Thank you in advance.