Kyut,
We presume you are performing the design per the British code - BS8110.
Within the STAAD environment, there are two modes for designing concrete members.
One is by specifying a set of commands in the input file. This is called the batch mode. The message that you have encountered occurs in the batch mode for the British code.
The other is by using the tab known as "Concrete Design". This is known as the RC Designer mode. This mode is designed to handle columns in tension. So, you may want to give this one a try.