iphone 6

iCloud uses Apple ID credentials.

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

Did you already try the troubleshooting mentioned in this related article?

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Follow these steps 

1. Restart your iPhone or iPad.
2. If you're using a cellular-data connection, connect to a reliable Wi-Fi network instead.
3. If you get an error message on your iPhone or iPad that says the activation server is temporarily unavailable or cannot be reached when trying to activate, wait a few minutes, then repeat the previous steps.

If you still can't activate your iPhone or iPad, try again using your computer instead:

1. Make sure that you have the latest version of macOS or the latest version of iTunes.
2. Check that your computer has an internet connection.
3. With the cable that came with your iPhone or iPad, connect your iPhone or iPad to your computer. Locate your device on your computer.
4. Wait while your computer detects and activates your iPhone or iPad: 

• • If an alert appears that says "Set up as New" or "Restore from Backup," it means your iPhone or iPad is activated.
  • If you get an error message on your computer that says that the SIM card isn't compatible or is invalid, contact your carrier.
  • If you get an error message that says the "activation information was invalid" or "activation information could not be obtained from the device," use recovery mode to restore your iPhone or iPad.

If you still can't activate your iPhone or iPad, contact Apple Support.

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copied from: If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support