As social media resource Twitter is designed to allow users to make and receive up-to-the-minute updates, the site exists in several mobile-based incarnations to make posting and read posts easier on the go. If you believe you've taken the correct steps to use Twitter from your phone but experience errors when you try to do so, retrace your steps and find out where you erred.
Faulty Internet Connection
Just as is the case when you access Twitter via a desktop and laptop computer, one prerequsite for using it on your mobile device is having an active, uninterrupted Internet connection. On most mobile devices, this exists in the form of a cellular data connection which derives from the same signal which enables you to make calls. If your reception is poor of if your mobile Internet simply isn't working, posting to Twitter can be difficult or even impossible.
Application Out of Date
On web-ready smart phones, two methods for accessing Twitter exist: navigating to the site on your phone's browser; or downloading the standalone Twitter application compatible with your device. Just as you must with software applications on your computer, you must regularly update mobile applications to ensure their proper function. If you're having troubles using Twitter, check your device's "Application Store" to make sure no updates for your device's Twitter application are currently available for download.
SMS Updates Configured Incorrectly
If you don't have Internet on your phone, it's still possible to update Twitter, namely, via SMS text message. In order to enable this functionality, you must activate the service on your phone. If you followed the steps to do this on Twitter's website and don't see updates you make from your mobile phone online, it's likely you configured your phone incorrectly. To start again, text "START" to "40404" and wait for the instructions, which will prompt you for your Twitter username and password. Make sure you enter these details correctly, as doing so will prevent you from updating Twitter via SMS.
If you can't find a reason your mobile Twitter updates are failing that has to do with you, it's possible the site is simply down, whether for scheduled or unscheduled maintenance, or simply because it's over capacity. If you have access to a computer, visit the Twitter website. If attempts to tweet --- or indeed, even to view --- the site or your own profile fail, the problem lies with Twitter itself, and not in anything you did.