i have an iphone 7 that is locked

There's multiple kinds of "locking" on devices. Click on the links below to find instructions on how to deal with your specific situation:

1) iCloud: Activation Lock - https://support.apple.com/HT201365 - This is a security feature so only legitimate (i.e., not stolen) redistribution of devices is possible. See: 'How to remove Activation Lock' --> https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it. Get them to remove it or return the item for a refund. If you are the owner, see the part at the end about asking Apple to reset the device.

2) Device lock: "If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled" click here --> https://support.apple.com/HT204306

3) Carrier lock: Read this link on "How to unlock your iPhone for use with a different carrier" - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.