I do not think it is possible to continue the stage after clicking cancel.Because a stage will not be triggered at all if the pre-deployment condition is rejected.
If the clean up task is necessary regardless of the success of the previous task, then it is a good choice to add it in previous task. This is logically reasonable.
Though, you can add a separated stage to perform the clean task which does not need pre-deployment condition.
[![enter image description here][1]][1]

[1]: https://i.stack.imgur.com/E10SI.png

I do not think it is possible to continue the stage after clicking cancel.Because a stage will not be triggered at all if the pre-deployment condition is rejected.
So, just as R Jain suggested in the comments, if the clean up task is necessary regardless of the success of the previous task, then it is a good choice to add it in previous task. This is logically reasonable.
Though, you can add a separated stage to perform the clean task which does not need pre-deployment condition.
[![enter image description here][1]][1]

[1]: https://i.stack.imgur.com/4eQrT.png