SIM900 Error on AT Command AT+SAPBR=1,1
I'm actually also working on a SIM900 and had the same problem.

One possible cause of the problem is that the SIM900 already has made a connection. Try re-starting the sim900 before issuing the commands.

Also I set the APN settings with the AT+CSTT= command

Below the in/output of my serial connection:

For testing purposes use this. Be sure to have the right APN settings, this will differ for other countries and ISPs.

StopSim900;
[SIM] Stopping
[SIM] Module disabled.
StartSim900;
[SIM] Starting...
[SIM] Module started
-----------DEBUG-----------//Pressed button 1 here. Initiates the HTTP get
AT+CSTT="internet","tmobile","tmobile"
AT Issued, response:
OK

AT+SAPBR=1,1
AT Issued, response:
OK

AT+HTTPINIT
AT Issued, response:
OK

AT+HTTPPARA="CID",1
AT Issued, response:
OK

AT+HTTPPARA="URL","http://www.minimalsites.com/"
AT Issued, response:
OK

AT+HTTPACTION=0
AT Issued, response:
OK

-----------DEBUG-----------//Pressed button 2 here, reads the http message
AT+HTTPREAD=20,10
AT Issued, response:
+HTTPREAD:10
&gt;Minimal S
OK

AT+HTTPTERM
AT Issued, response:
OK




I have left my answer here for further reference.