Can't find the error, because it gives the same answers as those stupid precalc solutions:

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int soma[40] = {0};

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, i, lim;
  ll ans = 0;
  cin >> n;
  lim = pow(10, n/2);
  for(i=0; i<lim; ++i){
      soma[i%10 + (i/10)%10 + (i/100)%10 + (i/1000)%10]++;
  }
  for(i=0; i<=n*9; ++i){
      ans += soma[i]*soma[i];
  }
  cout << ans << '\n';
  return 0;
}

  for(i=0; i<=n*9; ++i){
      ans += soma[i]*soma[i];
  }

i is out of soma range.