I think conceptually it's probably fine, but with only 65 observations you are asking your model to do a lot. As written, it's estimating a unique parameter (type = un) for each of the matrices you've specified (since time is a 3-level variable, those are 4x4 matrices). It sounds like the convergence issues you're having are because of this workload--there is perhaps just not enough information to do everything you're asking. If it were me, I'd start trimming away random effects that are not supported by the data. You could consider using some fixed effects instead of random effects where appropriate. This is a good resource for MLM: https://support.sas.com/resources/papers/proceedings13/433-2013.pdf Also see here for information about your error messages: https://blogs.sas.com/content/iml/2019/04/03/g-matrix-is-not-positive-definite.html Good luck!
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