After enabling the Twitter extension inside Flarum you need two keys:
To get this keys you need to create an Twitter app entry:
Go to https://developer.twitter.com
Login with your Twitter username and password
Now select your username at the top and "Apps"
Select "Create App" and filled out the form like this
A name for your app like "Forum", "Flarum Forum" or whatever you want
A longer description for your app. Write whatever you want. Not important.
Your website url where the forum is.
Sign in with Twitter:
Enable this. VERY IMPORTANT
Your flarum website URL and /auth/twitter at the end. VERY IMPORTANT
Terms of service URL:
A link to your terms of service. Not important.
Can be left blank. Not important.
Organization website URL:
The website url of your main website. Not important.
Describe what this "app" does. In our case to sign in with Twitter. Not important.
Go to "Permissions" and select "Request email address from user" (VERY IMPORTANT)
Select "Keys and Token" and you see your two needed keys in the area "Consumer API keys"
Enter this two keys inside the Flarum Extension for Twitter Login
Now try if all is working.
If not search the Forum or create a new thread for help.