There are times in some stories where the internal conflict is the main conflict, and can even be visualized as a fight inside of the protagonist's mind or turned into an external conflict. An internal conflict being the main conflict might even work better depending on what type of story you want to tell.
Yes, the main conflict can be internal. A major example would be the dude with the problem kind of story, and I personally love them. That's why I mentioned 'at a scene level', though I admit I messed up big time with what I actually wanted to say. Let's take another example:
Suppose the main character is an alcoholic who lost her parents recently. Her finances are in a dire situation, and she knows she can't turn back if she fails the upcoming exam. The main conflict is internal, but you can solve it by achieving external goals:
1. Stop being an alcoholic.
2. Get a job.
3. Pass the upcoming exam.
It is generally better if the goal isn't vague, like becoming happy, but what she has to do in order to be happy. Of course, it could be that what she thinks she needs vs. what she really needs are two different things. Maybe she needs a love interest to confide in about her trauma?
Anyway, so she starts by going to university again (external conflict), but she snaps at someone at a minor inconvenience for her bad temper (internal conflict making her life harder) and in turn discourages her from going to university again.
Returning home, she meets a friend of hers who is also an alcoholic and encourages (external conflict) her to have a drink. Due to her bad day, she agrees—if only to numb her emotions (internal conflict is stopping her from achieving the external goal).
The drunk protagonist goes for a job interview and, understandably, gets rejected.
In all of the cases, the scene goals have been external, even though the core conflict (person vs self) is internal, and worked to make the protagonist's life harder.