Hello,
There are 2 issues in your screenshots. The first one is that you have not added any fields to your export template. The list of fields you show a screenshot of is the option to sort fields, not the option to add fields. To add fields to your template go to Templates -> Template fields and select your template. After that click the New button to add new fields.
As for the Internal Server Error, please read the
500 Internal Server Error document. You will need to get the real error from your hosting provider.
Kind regards,
RolandD
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