Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Just a guess because I've never subscribed to anything through Roku ...
Did you take out your Sling subscription through Roku (as opposed to directly with Sling)? If so, perhaps you have to cancel the Sling-through-Roku subscription first before you can remove the Sling app from your Roku. Log into your account at https://my.roku.com/account/subscriptions where you can manage your subscriptions.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T