I don’t have the email address associated with my iPhone Apple ID. No one else is using the email address I want to change it to. However, Apple won’t let me change my ID. The message says the email address is associated with another account.

Get a new email address from AOL, Yahoo, or some other email provider and use that as your new Apple ID email address.

See -> If you forgot your Apple ID email address or phone number

If you forgot your Apple ID email address or phone number - Apple Support

-> Go to -> Settings -> tap your profile -> Name, phone numbers and email

You cannot change another person's Apple ID account/email address.

Have you bought a 2nd hand device?

Each Apple device is linked/associated with an Apple ID of the person who did the setup.

Activation Lock prevents anyone who is not the owner from using this iPhone.

To unlock the Phone, you need the Apple ID and password of the previous owner, that were used during set up.

You need to contact the previous owner.

Apple cannot help you.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock

How to remove Activation Lock – Apple Support (UK)

See -> The All Too Common SAD Reality of Buying a Used iPad/iPhone

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

You are welcome.