k/n is not a valid variable name. Use
steady_state_model;
A = 1;
k_n = (alpha/(1/beta)-(1- delta))^(1/(1-alpha));
w = (1 - alpha)*(k_n)^alpha;
R = alpha*(k_n)^(alpha -1);
r = R - delta;
n = (((1- alpha)/theta)*(k_n)^alpha/(k_n)*((theta + 1 - alpha)/theta) - delta*(k_n));
i = delta*(k_n)*n;
y = (k_n)^alpha*n;
c = n*((k_n)^alpha - delta*(k_n));
end;
Also, don’t do a full exp()-substitution. Go for auxiliary variables, see Question about understanding irfs in dynare - #4 by jpfeifer