It’s the model formulation. You need to make sure that the covariance matrix argument to your MvNormals is guaranteed to be a symmetric positive definite matrix. Using a multivariate normal prior on A and then using A as a covariance matrix won’t work.
Also, I can help you faster in the future if you strip down your code to the bare minimum - try to include only the essentials! For an example of how that would look, check this gist out.