I’m sorry you’re encountering issues with this plugin.
For other users who have experienced this behavior in the past, this is a symptom of one of two potential problems.
The first is a plugin conflict. Try disabling plugins and seeing if the problem still occurs. Especially any plugins or themes that manipulate or modify the WordPress editor (adding toolbar buttons, modifying behavior, et cetera). If disabling a plugin fixes the problem, let me know the name of it, and I may be able to circumvent the interference.
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If disabling plugins does not work, then, unfortunately, this is a manifestation of a rare bug that has interfered with Tweet This for some time. For some users (and sometimes only intermittently), the WordPress editor is not accessible programatically, like it is supposed to be. This means that Tweet This can not insert the shortcode automatically. For some users plagued by this, it is detectable and an error message results. For others, it’s not detectable and Tweet This fails. In your case, it would be the latter.
I have spent dozens, if not hundreds of hours trying to remedy this rare problem, but I can not reproduce it, and it affects a tiny percentage of users. This makes it very difficult for me to work on it. Nearly every plugin update for the past year has included something aimed to resolving this issue, but each attempt is a shot in the dark, and I’ve only managed to reduce the number of users experiencing the problem a bit.
If you fall into this second problem category, I unfortunately have no quick fix. You can tell Tweet This to display the shortcode for copying and pasting by changing the “Insert Shortcode” Button Behavior setting under the Tweet This’ settings page Shortcode Creator Dialog section. Setting Screenshot: https://1drv.ms/1VT9GMd. Resulting copy & paste dialog screenshot: https://1drv.ms/1VTa4dv. This will allow you to utilize the Shortcode Creator dialog box, but will involve one additional step after clicking “Insert Shortcode.”
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I’m sorry if the suggestions above are not helpful, or do not resolve the problem.