It seems like there might be a few reasons why you're having trouble casting your line into the fountain near the playground in the game.
First, ensure you are standing in the right spot near the fountain. Sometimes, the game requires you to be in a specific location to cast successfully.
Second, check if your inventory is full. If it is, you won't be able to catch anything, so make some space by selling or storing items.
Lastly, double-check that you have equipped the Seafoam Pudding properly. Sometimes, items must be equipped in a specific slot for their effects to work. If you've tried all these steps and still can't cast, there might be a bug in the game, and you could consider reaching out to the game's support for help.