Structured state-space model estimation fails

Following error/ warning message is shown once I try to estimate the state-space model parameters using ssest command.

Warning: Calculation of the goodness of fit measures failed because of the following error:

The numeric value of the "OutputWeight" estimation option must be a positive square matrix.

> In ctrlMsgUtils.warning (line 25)

In idpack.ltidata/computeModelQualityMetrics (line 223)

In idss/pem_ (line 95)

In ssest (line 144)

A quick system description,

9 states, 10 outputs, 6 inputs, and matrix D and K are set to zero.

Setting free parameters in matrix A and B to 1 and other known parameters to a fixed value of 0.

All entries of matrix C are fixed.

Any updates on why this error occurs and possible solutions are welcome.

1. Is model parameter initialization incrorrect?

2. do I need to specify advanced settings using ssestoptions?

3. Is any fundamental knowledge lacking before trying this approach for obtaiing state space model of a MIMO system?