Hi, can you not purchase tickets online for months in advance? It's February and I'm trying to purchase for April but it says none are available.

To purchase tickets please visit our website https://www.americandream.com/venue/nickelodeon-universe/tickets

I'm not sure, but I purchased my tickets online for the same day

So you can book up to 2 months in advance. Same day is also possible as well as next day, all the way up to 2 months. So depending on the date you want in April which I'm sure will be available, book end of February or in March.

You cannot purchase advanced tickets yet simply for the current day

I don't believe you can do that. You can purchase tickets online the same day you plan to go there. They have an online redeem ticket bracelet line.