Why can't Tasker manage battery mode on my rooted Pixel? I've been using Tasker to mute my phone at work. I recently added some nested tasks to enable and disable battery saver mode when I arrive and leave. However, it doesn't seem to have permission.
The phone is stock rooted Oreo on the October 2018 security patch release. Tasker is up to date.
I first see a notification:

Missing Permissions
Tasker needs the following permission(s) for the funcationality you  have configured:
Write Secure Settings

Tapping the notification, I then get a modal:

Missing Permissions
Tasker needs to have permission to write secure settings to use this action. Want to learn how to grant it?
NO OK

Then another:

Use Root
Since your device is rooted you can automatically grant this permission. Do it now?
NO OK

Opting for the affirmative results in a toast notification:

Tasker has been denied superuser permissions.

The battery mode never gets changed. Thanks for any assistance.
As it was confirmed in the comments, Tasker was denied superuser access in the Magisk app. Tasker can enable battery saver. 

If anyone wants a command-line solution, try this:

Code -> Run Shell -> Command: settings put global low_power 1 -> tick Root.

Replace 1 with 0 to disable battery saver via command-line. 

Tested on OnePlus 6 running Android 8.1.