Hello there, Thor.
It sounds like you are setting up iCloud Keychain and need to provide a mobile number for SMS verification. It looks like Cambodia is not listed for you because iCloud Keychain is not yet available in that country. The following Knowledge Base articles provide more information and updates on iCloud Keychain availability:
iCloud Keychain availability by country
iCloud: Frequently asked questions about iCloud Keychain
Thanks for reaching out to Apple Support Communities.