There is a bug in Steam's in-game popup system sometimes.
Here is a workaround:
- Go to Steam client, Settings -> In Game -> Enable the steam overlay while in-game. Make sure it is on.
- Go to Steam client, Settings -> Account -> Add Funds. Make sure you have enough money in your Steam wallet.
- Kill / Quit the Steam client, and restart Steam.
- Launch Starri -> Go to shop and buy the item.