Presumably you're suggesting that the character act as a third-person limited narrator for sections and the rest is in first person of the same character?
It's unusual, but not unheard of - and typically there's a clear narrative delineation between the sections. The Lovely Bones for example switches from third to first person upon a character's death (and the first person narration is from the deceased's POV) and it can be useful as a way of clearly showing separation of the "main" narrative from a framing device and can potentially even be required in order to make such a frame work (such as journal entries and so forth, a character writing about themselves in the third person in their journal is going to get very weird, very quick!).
As a guideline you'll want to make sure the shifts between the two are obvious and intuitively followed, especially if you plan on switching back and forth, but there are no rules about doing this or not doing it, as ever the driving force must be;
What does it bring to the story?
If it makes the story better then all well and good - since it's unusual enough to be potentially considered a gimmick I'd want to be darn sure I was doing to genuinely make the story better, to achieve something that wouldn't work as well as just picking one or the other. If you ever find yourself tempted to do it simply because you find it easier to write a given section of the book in a particular POV then nine times out of ten you're going to be better off fixing the underlying problems rather than duct-taping a bit on in a different POV.
Update - The OP has clarified the situation somewhat in a comment:
Okay, I have a YA Fantasy novel that is primarily told with one character narrating. However, when this character is not in the scene, or chapters, the book reads like it's written in third-person. And when that character is in the scene, it switches back to first-person with the mention of I, signalling that this character is back in the scene. Is that okay
There's nothing wrong with doing that particularly - and that's actually somewhat more common since you aren't using the same character from the first person as the third person narrator. Using the "I" as a switch indicator is reasonable, You mention switching on a scene - you may be best off keeping the switch to a hard boundary such as a chapter. This will keep it easy to follow, if you were swapping back and forth mid chapter it's highly risking coming off sloppy.