I didn't have the game installed, downloaded it, installed it.

No DLSS option. Downloaded the 3.51 patch from menu, installed. Same.

What am I missing?

I am in the process of redownloading (I uninstalled) to see if anything changes..which I will check out after work today.

I cant use DLSS yet so this is a bit of guesswork :

It won't work if ..
You don't have an NVidia RTX Graphics card, and or
You dont have the latest NVidia Graphics drivers installed, or
Maybe you didn't find the setting for DLSS (I think its supposed to be in the Anti-aliasing settings, but it wont show if you don't have both of the above)

If you can tick all those as present / correct then I don't currently know whats up there.
I do not have a machine with an RTX card so can't experience it myself yet.

Sorry cant be more help at the moment.

Assuming you have a compatible graphics card, and looked in the right place (it isn't listed by itself - it's under the AA choices) - assuming those - and still not seeing it - might need to update your graphics drivers.

That was my situation. My nvidia drivers were about 6-months old (don't recall the version); and I updated to the latest (of a few days ago), THEN the DLSS appeared.