i cooked a cheesecake and forgot to add the sour cream will it be ok?

I'm guessing it will affect both the taste and the texture of the cheesecake. It will probably be usable, but not as good as if you had added the sour cream. One way to avoid omitting ingredients in the future is to assemble everything you will need for a recipe on a tray before you start. Then, as you use each item, remove them from the tray and put away. If you end up with something still on your tray, you can generally fix things before you pass the point of no return.

What's the recipe you are using?

Probably ok. It's most likely there for a subtle tang. I'm assuming it was a small amount since you forgot to add it.