You need to install Ubuntutouch recovery.
TWRP cant handle ubuntu touch ota updates.
If you are using TWRP flash the update zip from github.
You can flash the zip file "with recovery" to get the ubuntu recovery installed but note that TWRP is gone then and you need to reflash with pc if you want it back.(maybe there is a method without pc, which idk. If yes someone can tell us here pls). There is no need to erase nand.
halium 11 ubports for herolte and hero2lte. Contribute to Kethen/samsung-exynos8890 development by creating an account on GitHub.
github.com
To make things easier you can use the ubuntu touch installer which now supports s7:
A friendly cross-platform Installer for Ubuntu Touch. Connect a supported device to your PC, follow the on-screen instructions, and watch this awesome tool do all the rest!
devices.ubuntu-touch.io