What model and model number Roku do you have (Settings > System > About will show you)?
Many of the newer channels will not run on older, less capable Roku models and can't be installed there. I can't find any information on which Roku models Discovery wrote their Discovery+ app to be compatible with.
The 3100 is a Roku 2 from 2011. It is a second generation Roku, while the current products are generation 8 and 9. It is no longer supported by Roku and no longer receives updates to the Roku software. The 3100 maxed out at Roku 9.1, while the current version is 9.4.
Many of the newer channels are written for the capabilities of newer units and more recent operating systems and can't be installed on older units.
I've long wished that Roku required the channels to list the Roku models they support in their channel descriptions.
Thank you for taking the time to respond to me. I knew it was an old unit and was time to upgrade. I have since purchased a Roku Ultra, hopefully that one will work. Be well.