I had the same problem and the TwitterKit documentation is not accurate or Twitter has changed their policies. In any case, on the Twitter Apps site, the callback URL has to be set very specifically. The callback URL should be in the format:
twitterkit-{consumer/api key}://
For example:
twitterkit-128238aKjqlp123AKdasdf://
Also, this needs to be registered in your Info.plist exactly the same:
<dict>
<key>CFBundleURLSchemes</key>
<array>
<string>twitterkit-128238aKjqlp123AKdasdf</string>
</array>
</dict>
More documentation is listed here: https://developer.twitter.com/en/docs/basics/callback_url.html
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