Correct spelling, explanation: personnel is a word of French origin, which looked exactly the same way. In English, you can clearly see the word person there, and the suffix -el is sometimes added to signify the tool (however bad that might sound). In any way, the word should stay in its original form which is personnel.

Definition of personnel:
noun
1. people employed in a facility
The hotel has very professional personnel; both the room service and the concierge were very thorough and kind.
This area is accessible to authorized personnel only.

Incorrect spelling, explanation: personnal is a misspelling of personnel, but it also looks like a wrong spelling of personal. In this case, try to avoid the -al suffix, as it is often associated with adjectives. Instead, try to remember that personnel is a French word (you can even read it with a French accent in your head), and base your spelling on that.

Incorrect spelling

Incorrect spelling

Incorrect spelling