iPhone to iPhone text bubble now Green??

I bought second hand iphone and the digit code, since I don't know the code I guess it, and the iPhone is lock, I see padlock on iphone screen, how do I remove it

Hello 22R3V16,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with sending iMessages. Follow the information here to resolve the issue: If you can't send or receive messages on your iPhone or iPad - Apple Support

"If you see a green message bubble

If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage. There are several reasons for this:

• The person that you sent the message to doesn't have an Apple device.
• iMessage is turned off on your device or on your recipient’s device. To check if iMessage is turned on for your device, go to Settings > Messages > iMessage.
• iMessage is temporarily unavailable on your device or on your recipient’s device.
• You might need to update your settings for Messages after you set up a new device.

Learn the difference between iMessage and MMS/SMS messages"

We hope this helps.

Cheers!