Try adding a small epsilon to the diagonal. See for more info Cholesky decomposition failed for precision/covariance matrix - #5 by anon75146577
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matrix[N_eta, N_eta] zeta_Lcov = add_diag(cholesky_decompose(zeta_cov), 1e-6);
Try adding a small epsilon to the diagonal. See for more info Cholesky decomposition failed for precision/covariance matrix - #5 by anon75146577
...
matrix[N_eta, N_eta] zeta_Lcov = add_diag(cholesky_decompose(zeta_cov), 1e-6);