Custodial account Issue

I am exercising following error message

"“To earn BAT, users must connect a custodial account to Brave Rewards. Unfortunately there’s no have a custodian available in your region, so earning isn’t available. For now, turning on Brave Rewards will automatically support creators.”

How could I create custodial account to connect with Brave browser to earn BAT?

Note: I’m choosing a country Sri Lanka"

You can’t. As it says, your region is not supported. You can see a list of all supported countries at https://support.brave.com/hc/en-us/articles/6539887971469

You just will have to wait for your country to be supported or for on chain payments to arrive later this year.