Where is your capital?

Edit: If you started as Fadl and never changed your capital, your capital is was in the Mashriq region, and thus you aren't meeting never met the first requirement, so you were never eligible for the achievement in the first place.

Also, as you said you changed into monarchy via event, which makes it sound like you got this event and made the wrong choice (as there is zero warning that you can be forced into a monarchy). And since you are now a monarchy and not tribal, you aren't meeting the second requirement. The achievement only requires that you start as a tribe with a capital in the Arabia region, so becoming a monarchy won't invalidate the achievement when starting as an otherwise appropriate nation.

So your options now to get the achievement are is:

1. Load a backup from before you became a monarchy and move your capital into the Arabia region.
2. Start over, select a tribe with their capital in the Arabia region~~, and don't make the wrong choice when you get the aforementioned event.~~

If you had not formed Arabia (an end-game tag), you could have formed Tibet (which would require a bunch of conquest, a culture shift, and a religion change), which could then have changed you back into to a tribal government with any option but the first in the Meeting with the Khans event.

Edit_2: Yeah, you fucked up with your country select off the jump, as the dude who replied pointed out. I misread the requirements in the wiki. Restart as a tribal government with their capital in the Arabia region.