So I made a Death Domain Cleric and I was wondering why I can't choose a Deity because I want to choose Jergal but I can't. So I don't get his Channel Divinity spells. I am stuck locked in with the preselected choices of Turn Undead and Touch of Undeath. Is there a way to change the Channel Divinity spells or to add extra spells so I can get the correct Channel Divinity spells?

A few things:

Channel Divinity powers are not spells and have nothing to do with spells.

All Clerics get Turn Undead as one of their Channel Divinity powers.

You don’t pick a deity as a Cleric, you pick a domain. There can be many gods of death, all that matters is the domain, not the god.

All Death Domain Clerics get the same Touch of Death Channel Divinity.