first of all if u are able to send the normal messages from the messaging app..then there is no problem with ur network and ur credit...in this case it is better that you first uninstall the app and do a reset and then install the app once again....
if u are not able to send the normal messages from the messaging app..then there is a problem with ur network or ur credit......which in turn is affecting this....
if u are facing this problem after changing the os version of ur 3gs then it is better that u update it to the latest version...or if u are facing this problem because of changing the network then i would recomend that u contact the network operator once....
reply if u have anything else....happy to help....