Let’s go through the checklist found in the official docs and try a few other things.

Initial check list

• While coping widgets from other form you changed the names. Right model, right field, right url.
• Your url must be reversable. Run shell in the Django console and then django.urls.reverse('your_url_name') – it must return something.
• You use ModelSelect2Multiple for ManyToManyField() and ModelSelect2 for ForeignKey()

Initial check list

• Your ManyToManyField() should not have a through table. There is no point in that at all.
• Your ManyToManyField() in admin should not be altered by being included in filter_horizontal. See the difference on the pictures (the first is without the field being included in filter_horizontal and the second is with)

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• Don’t forget to set form = YourForm in ModelAdmin inherited class for your model.
• In case you decided to alter the project’s urls, if you have separate ones for development and production, don’t forget to keep that in line 🙂