Cannot create iTunes Store account

You don't say where in your application you are trying to use the number.

Are you in India or China? Those are the places where you can use a number as an Apple Account identifier. Using your phone number for Apple Account instead of an email address - Apple Support

What is the exact error message that you see? Apple doesn't really say anything about when a number is denied so your best resource is to ask Apple directly. Refer to this page for Apple Support for different countries ➔

Choose your country or region - Official Apple Support

Select your country (if you do not see your country, also look for "other" in regions; e.g., "Other Africa"), then "Choose a product". Click the "show" item under each category for more selections. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

Did you follow the steps mentioned in this article? You can also select the device you are using to create it:

How to create a new Apple Account - Apple Support