ERROR on SIM900AT comand HTTPDATA (POST method)
If you say you are going to send 90 characters you must send 90 characters. No more, no less. I count 70 there.
You have to send them immediately the modem responds DOWNLOAD otherwise it times out (10 seconds) and you get ERROR. It looks to me like you're not sending anything (except maybe a blank line) for those 10 seconds and only trying to send once it has errored.