So.. i just bought 2 Character slot coupons,it shows 18/22,but when I'm about to create a new character I have 0 empty slot. I deleted my other 2 Characters a couple weeks ago so according to that I still have 2 empty slot right?
But it's gone.. does anyone know what happened to my other 2 slots

yeah. I have also experienced the same issue. Have you tried finishing the settings of characters that you created by using Recruit Ally items? If this this did not fix your issue, then I suggest sending ticket to support. 

"Have you tried finishing the settings of characters that you created by using Recruit Ally items?"
It had nothing to do with this... you can save/delete them - if you have no idea why answering?

I hope when the update come out it will fix my issue

Still missing, need fix asap

If you used the New Ally in the story you have to go back and finish setting them up. Then for some reason you'll see your slots. This was the answer from the BDM discord.

Can confirm this worked for me. Yes you can delete them but guess there is a bug tied to it who knows.