How to increase the frequency of data transmission of SIM900A GSM Module connected to Arduino Uno?
There does not appear to be a complete description of how ThingSpeak throttling works.  However there is this text on the Thing Speak License FAQ web page:

As a reference, one unit provides the ability for a device sending
data to ThingSpeak at once per second to send data to ThingSpeak for
one year.

Assuming your code is working as expected and the ThingSpeak throttling is to suspend your service for 15 minutes, it may be that you are sending data faster than ThingSpeak will allow.
To test this, try to send messages slower than once per second.