The plugin works for new posts on save. for pre-existing posts you need to edit and save.
If you create a post and save it, the tags (already existing tags) will be added, 100% sure.
Whe you click ‘Publish’ the most recent versions of WordPress (using the new editor Gutenberg) say:
‘Are you ready to publish?’
‘Double-check your settings before publishing.’
‘Suggestion: Add tags’
This is for manually added tags. The tags added by the plugin won’t appear here just because the post is not saved yet.
Please double check your settings and the FAQ, unless you consider it a disappointing waste of time as well: