i reset my iphone 6 plus to factory settings and i tried activating it again but it gives an error and tells me to contact apple support , i have tried almost everything , please help me .

If your device has an activation lock, only the original owner can unlock it using his credentials. There's no way to bypass the activation lock, and that device is useless.

If you are the original owner of that device and you forgot your Apple ID (and/or its password), follow the steps provided in these support guides:

If you forgot your Apple ID - Apple Support

or

If you forgot your Apple ID password - Apple Support

If nothing helps, contact Apple Support directlly using this link:

Contact Apple for help with Apple ID account security - Apple Support

Regards

Giulio