Answer: You just need to follow the instructions step by step. Please don’t skip anything.

Please see #4 and #5 for Twitter:

4. Click “Permissions” tab. Change Access level from “Read Only” to “Read and Write“. Click “Update Settings”.
5. Click “Keys and Access Tokens” tab. Scroll to the “Your access token” and click “Create my access token” button. Refresh page and notice “Access token” and “Access token secret”. Make sure you have “Read and Write” access level.