It's either bad programming on Twitter or instagram.
But on Android phones if your twitter password uses special characters the instagram app will not be able to login to twitter. So to get instagram to link to twitter you first have to change your twitter password to simple numbers and letters (although twitter fully supports special characters)
Then go back to the instagram app and link the accounts, this gives instagram permission to access twitter. Once you've done that you can go back to twitter and change your password to something that uses special characters if you prefer to have them in your passwords.#instagram #twitter #android #trouble #login #linking #accounts