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It is not possible to add comment to just added and unsaved content.
Cause
We use standard REST to create/edit inline comments, created by Atlassian. The mechanism of inline comments creation is following: the phrase that we have selected is searched for in the content of the page, but since the content is new, then in fact it is not existing in Confluence and therefore an error appears.
Resolution
Unfortunately, this feature is not available in the app. You can save the content first, after that comment adding will be available for the content.