As you can see in the screenshot below I have $14.81 gross revenue. But when I try to get payout it says I have $2.10. I don’t understand how that works and what should I do.
Yeah, it looks like 2 of your payments haven't passed the 7-day threshold yet. Wait a few days and then check back.
Keep in mind that the number Itch.io shows you on the Payouts screen is after credit card fees and your revenue share cut are taken out. It might also include taxes if you're outside the U.S., I'm not sure about that part.
The Stripe/PayPal fee alone is something like $0.30 + 3% so if your payments are only $1 then you're going to be losing 33% just from that. That's not Itch's fault, it's just the standard cost of processing credit cards (which is why some mom-and-pop stores have a $5 minimum for credit card transactions). Consider raising your price.
If you go to https://itch.io/dashboard/purchases you'll be able to see all the transactions and how much they were. There's a link to export a CSV (spreadsheet) which will break down all the fees and such.