Should I explain my "NP" grade in my statement of purpose?
This sort of thing is always a strategic decision. While some readers will appreciate the clarification, for others it may serve to highlight something that would otherwise not have attracted attention. I suggest seeking advice from professors at your current institution, who can comment on how your overall application looks.
If you do decide to address it, some options include:

Ask one of your letter-writers if they can deal with it in their letter;
Add a standalone note at the end of your Statement of Purpose. Don't work it into the prose of your SoP, just put something at the end such as


N.B. The 'NP' grade shown on my transcript for ... arose because I was unable to attend the final exam for personal reasons. I retook this course in .... and received an 'A'.


Include a cover note with the transcript.

Good luck!