Here is my app's code:
However, I get this error:
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From the documentation:
Apps must have a registered Stack Apps post to write.
The next sentence (about links pointing back) is no longer true, but the one after that is right and important too:
You can add or change your app's registered post from the Stack Apps App Management page.
Since this question is tagged app, you can use this one as the registered post, even though officially you should use another one. For testing purposes, it's fine I guess.