Hello, I need your help. I bought a phone that is locked and I want to open it on iCloud. It is locked

You will need the previous owner's assistance to unlock it. There isn't an option to bypass activation lock without that.

If you can't get assistance from the previous owner, if you bought it through a 3rd party (e.g., eBay), see if they have any buyer protection that can give you a refund.

Also see: Activation Lock for iPhone and iPad - Apple Support