If this was a previously owned iPhone and that owner was banned from Snapchat there is nothing you can do about it. Snapchat is the only social media service that actually bans devices, not just users.
Snapchat is very strict about enforcing its Terms of Use. This link explains their procedures and enforcement strategy→My Snapchat Account is Locked.
But basically, When Snapchat bans a user they also ban the device that was used to access Snapchat. If it wasn’t a permanent ban your account will be usable after the duration of the ban. Otherwise you will never be able to use Snapchat again from the device that was banned, and if you try to access it with the same account or information that can be associated with that account (such as name or phone number) from another device, that device will be banned also.