Because you want a random selection, you will need a script.
My script store includes a script that links to a random record. You will need a pro subscription in order to run the automation script, but the apps script can be used with free bases. In your [Tasks] table, you will need to create a filtered view of tasks without rewards, and set the linked record field in the [Rewards] table to limit selection to that filtered view.
Note that this script will link only one task to each new reward record. If you need to link multiple tasks to the same reward, you will need a different script.