It is impossible to play Realms due to you not being able to click the X button so you can play. I have tried closing the application as well and it doesn't work. There is no X button to close the buy realms tab and clicking on a option doesn't close it either.
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Unless you are certain that you currently either own a realm or are invited to a realm, this is a duplicate of REALMS-2288. According to this comment on REALMS-2288, the pop-up appears when you neither are invited to a realm nor own a realm.
This might only be for new accounts I am not for sure as this is my first account.