SAS: create covariance matrix with variables

I am working on a project in which we have to do some simulations. I am now at the point I want to use the RandNormal function, to generate multivariate normal data given a vector of means and a covariance matrix. For the covariance matrix, we have to use different parameters, so instead of putting the values in directly, I want to just put the variable name in the matrix (so we do not have to do the calculations by hand).

We get this error: ERROR: (execution) Character argument should be numeric. And this is our code. The dataset Simulatie also contains the variables defined in this proc iml.

proc iml;
use Simulatie;
read all;
close Simulatie;
sigma0sq = 2;
sigma1sq = 3;
sigma2sq = 1;
sigma0 = sqrt(sigma0sq);
sigma1 = sqrt(sigma1sq);
sigma2 = sqrt(sigma2sq);
rhoM = 0.7;
rhoV = 0.5;
rhoMsigma0sigma1 = rhoM*sigma0*sigma1;
rhoVsigma0sigma2 = rhoV*sigma0*sigma2;
rhoVsigma1sigma2 = rhoV*sigma1*sigma2;
Mean = {0, 0, 0}; /* population means */
Cov= {sigma0sq rhoMsigma0sigma1 rhoVsigma0sigma2, /* population covariances */
rhoMsigma0sigma1 sigma1sq rhoVsigma1sigma2,
rhoVsigma0sigma2 rhoVsigma1sigma2 sigma2sq};
N = 185; /* sample size */
call randseed(123);
X = RandNormal(N, Mean, Cov); /*x is a nx3 matrix */
SampleMean= mean(X);
SampleCov= cov(X);
quit;

We tried putting in for instance

Cov= {3 2 1, 2 4 0,1 0 5};

Which works perfectly nice, but when we change it to variable names, it gets an error. We did some googling but couldn't find the solution, could anyone help us out :) It seems like such a silly, easy-to-fix error, but we just don't succeed!

Thanks in advance.