In one of our environment, we can't drag the content fragment into the page, it returns
Does anyone know why I am getting this error?
Never seen this issue.
Can you test it in the fresh instance? If the problem still persists then, please share the steps to reproduce it.
And also share the error.log.
Try going through this article on a fresh AEM instance - Adobe Experience Manager Help | Working with Adobe Experience Manager Content Fragments
Let us know if you experience any issues.