Greetings, hamim2020!
Thanks for reaching out to the Apple Support Communities! It sounds like you are having an issue with using Keychain in Bangladesh and I am happy to assist. As you can see if this article with information about iCloud Keychain availability by country and region, Bangladesh is currently not a supported country for SMS verification, so you will need to follow the following steps from the article to continue the Keychain set-up:
Set up iCloud Keychain without an SMS-capable phone number
If your country or region isn't listed above, SMS verification isn't available in your country or region. You can still use iCloud Keychain by following these steps:
On your iPhone, iPad, or iPod touch
- Go to Settings > [your name] > iCloud, and tap Keychain. If you're using iOS 10.2 or earlier, go to Settings > iCloud, and tap Keychain.
- Enter your Apple ID password.
- When asked if you want to use your device passcode as the iCloud Security Code, tap Create Different Code.
- Tap Advanced Options, then choose one of these options:
- Get a Random Security Code.
- Don't Create Security Code. If you don’t create an iCloud Security Code, then your iCloud Keychain is stored locally on your device instead of Apple's servers, and will update only across your approved devices.
- Follow the onscreen instructions to complete iCloud Keychain setup.
Take care!