First of all, welcome to Feral-Heart! Secondly, how long is y'er username? The game's username limit is 16 characters. If y'er user exceeds that limit, 'twon't let ye play the game.
Also, be sure to give the game a few minutes to recognize that ye've logged in to the Play Game tab. If ye log in b'fore it registers this, then ye will 'ave to close the game and reopen it. The game won't log in afterwards, even if y'er logged in to the tab. Why this is, I don't know.
Make sure to check y'er internet connection. If y'er wifi is slow, it may not register that y'er logged in to the tab.
Hope this helps!