Did you power down all of your STEEM POWER (for example to cash out)? There is a glitch in the system where once you power down entirely, you can't transact at all (SP is used to allocate bandwidth in place of tx fees). In fact you can't even power back up since that too would be a transaction. The only way to recover is to get some SP powered up directly from another account, which can't be done through the GUI (only command line)

If this is the case PM me with your account name and I'll power up a little bit into your account so you can use it again.