You can't merge Apple IDs. You can try setting up Family Sharing between the IDs if you want to be able to share purchases made with the IDs.
Family Sharing. *
ASC YouTube for Family Sharing
Family Sharing - Set up. *
Family Sharing - Set up on iPhone. -
For the ID with the email account you can't access, you can change the email address. Note the restrictions in the article.
Apple ID Name Change. *
Make sure you turn off Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support * I suggest doing this before you change your Apple ID or password.