Reactions can occur on your own turn, but only in response to appropriate triggering stimulus
As the M&M rules describe the reaction action type:
A reaction is something that happens in response to something else, like a reflex. Like free actions, reactions take so little time they’re considered free. The difference between the two is a free action is a conscious choice made on the character’s turn to act. A reaction can occur even when it’s not your turn to act. Some powers and other traits are usable as reactions.
As it notes that reactions can occur even when it's not your turn, it's evident that reactions can also take place even when it is your turn. However, there is no provision for "reacting" to nothing at all - the text is clear that a reaction is a reflexive response to stimulus, as opposed to a conscious choice to act.
Some mechanics do say that characters "may" do something as a reaction, in which case they do have a choice about whether or not to respond to a trigger. The reaction power modifier however specifically states (emphasis mine):
Each effect has a default action required to use it: standard, free, or none (for permanent effects). See Action for details. This modifier changes an effect’s required action from a standard or free to a reaction, occurring automatically when a specific triggering event occurs. This is similar to the Triggered modifier (later in this section) but reaction effects do not need to be “set,” originate from their user, and can function repeatedly, so long as their triggering requirement is met.
When the triggering circumstance occurs, the effect activates automatically, even if it is not the character’s turn. The user can only prevent the reaction effect from occurring by choosing to deactivate the effect entirely (as a free action), in which case no circumstance will trigger it. For the ability to choose to have some triggering circumstances activate the effect and some not, apply the Selective modifier as well.
So a Reaction power is barely actually under the character's control in most cases, and it can only be used so long as the trigger requirement is met - if there is no trigger, the reaction does not happen. As written, even the Selective power modifier only affords them the ability to control whether or not they react without suppressing the power entirely; it doesn't grant them the option to use the power in the absence of a trigger.
The Variable power also specifically notes:
... only a Reaction Variable can change more often than once per turn, and then only in response to its triggering circumstances.
So in the absence of an appropriate trigger, you can't change up your Variable power.
You have options, though!
If your character wants to make use of a normally reaction-based power without the appropriate trigger, they could consider using a Power Stunt to temporarily gain an Alternate Effect - which could just be the original power without the Reaction extra applied, allowing them to use it with the power's normal action. As a power stunt, this represents the hero making an extraordinary effort to control a normally unconscious power.
If you envisage your character routinely both using their power deliberately and as a reaction, you could buy the Alternate Effect extra permanently and have one version with the Reaction extra and one without. You'd have to swap between which version was active with a free action (at most once per round), the end result narratively sort of being that the character can't use the power reflexively while they're concentrating on using it actively, which makes sense.
