According to the documentation of Add-PnPDataRowsToProvisioningTemplate for the parameter fields:
The fields to retrieve. If not specified all fields will be loaded in
the returned list object.
So, if you leave the -fields parameter empty, it will load all fields including default system fields like Created, Created By, Modified... These default fields are generated by SharePoint and could not be added/edit manually. I think that's the reason that would not work.
So you need to specify the fields parameter which can be edited by end users. Like
column duedate, Title, startdate
in the command.
Note: you need to use the internal name of the column, for the description column in the task list. The internal name is Body.
You could change field in the template <FieldRef Name="Description" />
to <FieldRef Name="Body" />
. Maybe it would work fo you
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