You can’t delete certain topics, those that are automatically created by Discourse, like the “About this category” topics.
I created a topic on my (up-to-date) instance, replied, and could delete the topic.
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Can you give us examples of topics you can’t delete?
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Are you admin or moderator?
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What are the category permissions?
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Did you try in safe mode?
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Are there errors in the js console, or the network tab of your browser’s dev tools?
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Are there errors in
/logs?