Mention the temperature in the keyword line e.g., Temperature=300.0.

Dear Sujan,

In window of additional keywords write order " Temperature = a suitable value " and check the updating and then run your calculation.

Regards,

Dear Sujan:

as said Dr. Mandal you must specified in your Z-matrix the temperature at you want to optimize

Regards,

Another option is to run the freqchk program of Gaussian in a job that is already finished. With this program you can calculate the thermochemical energies (or something else) at different conditions (temperature and pressure) performing only one frequency calculation on the geometry (or opt+freq).