There is already a plex account under this email address.
It has been deleted, but that was less than a month ago.
An email address cannot be re-used for 30 days after the deletion of a plex account linked to that address.
This is shown to the user when s/he deletes the account.
This question is not directly related to this thread but I do not think it deserves its own thread:
Can a user that wants to delete and recreate an account change the email on the first account and then delete the account and then create a new account using the original email?
This came up in a conversation with an elderly user I help from time to time. The reasons he wants to do that are not really important but he thinks it is the best way (from his point of view the only way) to accomplish what he wants to do. I tried to explain other alternatives but once he sees a path to a goal it is nearly impossible to get him to see a different route. As I said he is elderly. He is one of the very few people I know that makes me feel young.