I cannot download the discovery+ app from my Roku device or from my Roku app. Every time I try to download it it just errors...If I log into my Roku account it shows that I have the discovery+ app but I can’t play it on any of my devices or see it on any of my devices...
You can add anything you find on the Roku Channel Store web page to your Roku account. A channel will only download from your account to a device supported by the channel app, however.
I can't yet find any info specifically on which Roku models Discovery wrote their Discovery+ app to support, it's probably too soon, but here's what Discovery Go says it supports so Discovery+ may be the same :
Roku
And, although it's not listed, if it runs on an Express, I think it should also run on a Premiere.