How can I turn on Adventure Sync?

I have iOS 13 but I did set this up before I updated my phone so I'm not sure if I was just grandfathered in. Try the following in the Health app:

1. Hit the icon in the top right corner to go to your account.
2. Under the privacy section hit apps.
3. You should see Pokemon GO there.

Then just allow access to everything as suggested in the instructions from Pokemon GO.

EDIT:

Per the conversation in the comments, follow the directions in this Reddit thread https://www.reddit.com/r/pokemongo/comments/c58uo5/adventure_sync_on_ios_13_possible/ to fix this issue in iOS13 if the above did not work:

1. Turn off automatic app download under General in the watch app (if you have an apple watch).
2. Reinstall Pokemon GO.

If Pokémon Go is not listed under the Health options in your settings, you have a couple of different options to try based on a few sources:

1. Niantic Support suggests to check your Motion & Fitness configuration within your Privacy settings.

   It might be possible that Fitness Tracking is not permitted on your device, hence you're unable to view Pokémon GO in the Apple Health app. Try enabling it from the Motion & Fitness option under 'Privacy Settings'.

2. If the the problem persists after checking your privacy settings, the reddit community suggests un-installing and re-installing Pokémon Go. Many of the users there confirm this fixed this issue for them.

If neither of the above worked, then I suggest submitting a support ticket to Niantic