Seriously, if I don't have permission to post, don't give me the input form.
The reason is probably that the system found something "bad" about your post, see https://docs.microsoft.com/en-us/answers/questions/30752/access-denied-error-when-posting-content.html for a longer discussion.
That is not to say that your post was really bad, only that the system thought so. I run into this myself occasionally. It's a bit of guessing game what change to appease the system.
Share your feedback, or help out by voting for other people's feedback.
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