I don't think we can help you because you haven't provided any of the required Information.
You are sending AT commands to the Software Serial and a SIM900A module. The SIM900A appears to come as a shield or as a module which you connect with wires. Which are you using? We can assume a shield would be properly connected, if you are using wires ... have you attached TX to TX or TX to RX? How is the SIM900 powered?
You are sending to the SIM900 but not reading anything back? Why not? AT commands usually have a Response.
Looking at the datasheet for the SIM900, page 2:
"2.2 Command line
Commands always start with AT (which means ATtention) and finish with a character."
Your code:
SIM900.print("AT+CMGF=1\r"); // AT command send SMS
delay(100);
I don't see any CR...
I have never seen AT commands with a lot of spaces... But I will admit limited experience and a lot of years since then...