How can I create a reward where the player doesn't receive a physical item

To create such a reward, you can define it as "virtual" during the configuration process. Setting the reward as virtual means that it will be used for display purposes only, and the player won't receive any tangible item. Instead, you can utilize commands to provide benefits or actions to the player.

For example, if you want to give your players a voucher or give in-game currency, you can define the reward as virtual and include a command that grants the desired rewards or performs specific actions.

Additionally, you can use PlaceholderAPI to set random quantities for these virtual rewards.