Hello, You need to be cautious when increasing the random parameters in the model. Based on the error that you got, the variance-covariance matrix might be showing negative values for variance or 0 for covariance. This means a simpler model (without a random intercept/linear trend) is a better alternative in your case. Also, I don't understand why you used 'rep' (i am assuming this to be the id variable) in the model as a covariate. Thanks, Rajesh.
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