Hello, so I need help.
So I’m an artist who mainly draws with Japanese style and only started commision for one big work and one wip for convention (So in other words I’m very inexperienced)

But then there’s this person who came out of nowhere (and knowing my phone—I believe he might be related to my family’s acquaintance since I never give my phone number recklessly and mostly only give emails and such), asking for a comission for his business.

He wanted me to draw 20 drawings and for some reason, insisted that I should be the one who draw it for him. But then what he asked is somekind of Micky Mouse style which I never have experience on. Sure, I can learn to do it but I feel uncomfortable since I didn’t even state that I open commision. I am also scared that due to our desired style is different, I may screw things up.

Should I accept the job offer or no?

I'm an artist as well who does commission and if I was in your position, I personally would decline. Don't allow yourself to get pushed around. It's of course very great to do things out of your comfort zone, but that should be on your own time, so that you are able to learn and grow without any worry of a deadline or unhappy customer.
I would explain to them that you are uncomfortable with their request because you don't have experience in the style and your commissions aren't open. Tell them you'd be willing to take a different request in your own style once you open commissions.

Thank you so much for your words!! <3

For safety reasons, I suggest you also ask how he got your contact and perhaps why he suggested that specific style when you draw jn a different one.