You can take a bonus action if you have something to use it on

Consider the text for [Bonus Action][2]Bonus Action:

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action. The Cunning Action feature, for example, allows a rogue to take a bonus action. You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action. You otherwise don't have a bonus action to take.

If you don't have features, spells, or other abilities which let you take a bonus action, then you don't have any bonus actions you can take.

Consider the text of [Cunning Action][1]Cunning Action:

Starting at 2nd level, your quick thinking and agility allow you to move and act quickly. You can take a bonus action on each of your turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.

Cunning Action lets you can take the Dash, Disengage, or Hide action as a bonus action. Cunning Action is not "granting" you a bonus action "slot", it is giving you options which you can perform as a bonus action.

Spells with casting time as 1 Bonus Action give you another option. You can take that spell as a bonus action.

The text "You must use a bonus action on your turn to cast the spell" means that you are using your bonus action for that turn. You do not have a bonus action available if you have already used one on that turn. [1]: https://www.dndbeyond.com/classes/rogue#ClassFeatures [2]: https://www.dndbeyond.com/sources/basic-rules/combat#YourTurn