That particular error is probably coming from the fact that _change_reason is not an official field on the Model object, but rather just a special attribute that django-simple-history uses to fill in the change reason on the history model. That said, I also don't know how to allow change reason to be specified in the admin panel. I think it may just not be a supported capability at the moment. Would be useful though for sure.

I think that this feature would be awesome! Did you guys figured out a way to do this?

Edit:

I've just found a way.

forms.py:

from django import forms

from app.models import MyModel

class MyForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = "__all__"

    change_reason = forms.CharField(max_length=100)

In your admin.py, override save_model method:

    def save_model(self, request, obj, form, change):
        obj._change_reason = form.cleaned_data["change_reason"]
        super().save_model(request, obj, form, change)