Describe the bug
I click on Install using the Twitch app, which works just fine for retail TRP3, and twitch opens but nothing happens. I'm not redirected to the installer within the twitch app. Also, I can't find TRP3: Classic anywhere under the 'Get More Addons' tab, I can only see the regulat TRP3.

Version of the add-on:
Total RP 3: Classic

Did you try having Total RP 3 as the only enabled addon and everything else (especially something like ElvUI) disabled?
N/A

Make sure to select the Classic folder in the dropdown in the top right corner when managing/looking for addons to be able to install addons on Classic.

If the Install button on CurseForge doesn't properly work to install the addon with the Twitch App, that's an issue to open with them : https://help.twitch.tv/s/contactsupport under Twitch App