Why I’m not able to active wallet app in my country Oman ?

The Wallet app on an iPhone does not need to be activated; it's used by transactional cards for many purposes. One of these is to house your credit or debit card credentials for Apple Pay.

Are you asking why you can't use Apple Pay in Oman? If this is your question, then be aware that Apple Pay is not yet available in Oman. Apple publishes a list called Countries and regions that support Apple Pay which is updated regularly when new territories are added to the Apple Pay service.

Apple will advise users through this list when Apple Pay becomes available in new territories. This is also reported in the tech press when it becomes public knowledge.