T16 Toxic Sewer Not showing up on Map

have completed map a few times now and not showing up on map or in the map stash/tab.

Do you mean it's not showing up on your atlas? For it to be T16 on your atlas or in map stash tab, you need a certain number of Voidstones equipped on your atlas.

Yep, you need enough Voidstones for Toxic Sewer to be T16 on the Atlas. Its natural tier is like T6 right now. You should need at least 3 stones, or maybe you need all 4. Pretty sure it's 3, but I'm not near my gaming PC at the moment to verify which.

For stash, you need to actually put a T16 of that map into the stash for its slot to show up. It will do so automatically once you put one in. Completing the map or just having it on your Atlas won't add it to your stash.