Sweep can be performed only after completing one auto-repeat/auto-battle with 3 stars after you have reached the required Guardian Camp level. Once the "Auto" tag appears next to the dungeon you cleared, you will be able to use the Sweep function.
If you enter the auto-battle and your characters are not automatically fighting, please do the following:
1. Tap on the "Auto" button at the bottom of your screen
2. Exit and go back to the lobby
3. Restart the Auto Battle and it should work correctly.
4. Allow it to finish without interrupting it so that the "Auto" tag is added on your run.
5. Use the Sweep function.
For Awakening dungeon Sweep, please note that as each day of the week is a different "rift/dungeon" and you need to clear each of them at least once on auto battle, before being able to use the Sweep function.