Usually you buy food, not energy directly. You find that in your inventory and there is no time limit at all to use it.
Personally, I would suggest to never to buy food with gems, as there is so much better use for the gems. If you buy equipment, that will keep on giving energy, food is gone quickly.
Or buy shop events for gems, that can give equipment, material and food…
The offers have a time limit (that is displayed and clearly visible, usually one week) and with the next offer comes the next variety to choose from.
There is one issue with purchases, I will post a link with info in a moment…
See if that helps you and check your inventory for the food.
Help on how to contact pixel support is also within the link.
If all else fails, they can check and fix it for you, as they have access to your game.
Here we write in the public player forum and we fellow players don’t have that access.
Edit: I remove the bug report for now.
Should you get verification by pixel support that there was indeed a bug on pixel‘s side, please report that here and we will tag accordingly. In most cases players find the items in their inventory, sometimes after the procedures described in the link.