Issue Adding Sidekick

I am DMing a campaign for my buddy using the Dragonlance: Shadow of the Dragon Queen adventure module. He is the only PC and is using 3 sidekicks that came with the module to round out the party. I purchased the content both digital and physical through D&D Beyond. When attempting to add the sidekicks to the character sheet I have been going to:

character sheet > extras > manage extras > add an extra > sidekick

If I uncheck the box labled "Filter Using Sidekick Rules" it shows lots of monsters that can be added. But no sidekick options at all. Not from the adventure module or from Tasha's. Am I doing something wrong?

That's because they aren't added as monsters on the site. That search only pulls from monster stat blocks. 

You can build them up as homebrew options, which allows you to level them up as needed, though they'll need to be removed and re-added as you do.

Bizarrely, there are a number of categories in that "Extras" tab that refer to things the character sheet doesn't actually support at all; Sidekicks is one of them. It would be nice if they just removed all of that stuff so it didn't confuse people, but they...haven't.