This is a small bug in Pumas. I’ve filed an issue so it should get fixed before the 1.0 release.
Please notice though, that specifying a distribution currently won’t be different from specifying a domain when fitting with Pumas.FOCEI(). Eventually, we might make this do MAP estimation but currently the prior is only used for determining the domain. Hence, you should be able to successfully fit the model and call infer if you use the version suggested in Unable to interpret the fitting error