I've tried it out of curiosity, it works for me. I had a link where they recommended clicking though, to fix it and to make it work. I don't see it on your screenshot. I think I got a (slightly different) warning with a link when I tried to open Silence as an app. Regardless, it was essentially to open Settings > Apps > All Apps > find Silence > click somewhere (I forgot where exactly) to enable the potentially very unsafe thing of full SMS access. I think it was on the top right where I needed to click in the last step. Afterwards, Silence can be selected as the default SMS handler. That being said, I personally mostly use Signal and Matrix nowadays. The phone lasts for.. 2 days probably? IDK. I'm not really happy with either messenger BTW, but it's the best shot for now, for me