my iphone can't dial ussd code

i'm not sure, but make a back up in iCloud first do this:

How to back up your iPhone, iPad, and iPod touch - Apple Support

Than try this:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

If this doesn't work, contact Apple Support.

USSD (Unstructured Supplementary Service Data) codes are used and understood exclusively by GSM cellular data networks. Your cellular carrier dictates which codes are used and understood. Have you checked with your carrier? My USSD codes work just fine with my iPhone and my GSM carrier.