Secure Settings is no more
Secure Settings on Marshmallow devices is a pain to use. This essentially affects all devices with a system-less root access. No other methods of fixing the app or the root access were working for me. The app itself has been dormant with the updates since 2014, so I took it to google to find some solutions. Please note that I don’t use the Secure Settings as much as I used to, but sometimes I need an odd command to be available to me.
I was able to install the Secure Settings on a Nougat device (Nexus 6P) I believe the same method would work with the Marshmellow devices. Please see this article.
Replace Secure Settings with Shell Commands
You can find 40+ Shell commands in my other post here. Turns out that thanks to the Reddit post a lot of googles can be achieved with the shell commands. Root will be needed, however, I assume that you have one. If you are interested in using Secure Settings in the first place. as the Reddit post could be done little better, and more usable, I would organize the list in a more usable form.
Paste the appropriate command into Run Shell action in Tasker (Code – Run Shell), make sure the Use Root is selected.