Every day at 1am CT, Pokémon TCG Pocket resets its daily Poké Gold usage limit, where you can spend a total of 720 Poké Gold on opening packs and other activities before the game locks you from using any more.
You can still buy more Poké Gold in-game and store as much as you want, but the game stops you from using any of it. You can sit on thousands of Poké Gold (up to 3,200), but once you hit that 720 Poké Gold cap. That's it. No more opening packs, no matter how badly you want to try and complete a set, until tomorrow, unless you have Hourglasses to use instead.
