bought a iphone that was locked

if with a passcode then read

If you forgot your iPhone passcode - Apple Support

if activation lock then you can't

activation lock is not a thing on the phone it's a theft protection on apples servers

it needs to be removed How to remove

Activation Lock - Apple Support

if you don't know your appleID password If you have

forgotten your Apple ID

password – Apple Support (UK)

then you will need the assistance of apple Activation Lock - Support

(apple.com)

and or

Apple Support

And or

Contact - Official Apple Support

and or

Genius Bar Reservation and Apple

Support Options - Apple

If you bought a used iPhone, which is activation locked, the ONLY person who can unlock the phone is the person who put the lock on the phone. Apple won't unlock it for you. We can't help you unlock it. Contact whoever you bought the phone from and make sure they do what is detailed in this link --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you don't know that person or can't reach that person, you have bought yourself a useless device you'll never be able to use. And if that is the case, either try to get your money back or if you can't, then recycle the phone responsibly.

Read this --> The All Too Common SAD Reality of Buying … - Apple Community