Spectrum (FFT) display of signals from TBS2000B with the Tekscope software

Hello everyone,
I would like to display my signal from the TBS2000B oscilloscope with the TekScope software on the laptop using the Math function in the spectrum (FFT).
I get the following message: Invalid Signal Input. 
What setting do I have to make to display the signal in FFT on the laptop?

Many thanks for your help
 

Hi Gislain,
I was able to get an FFT with the TBS2kB on Tekscope; could I see the settings you have when you click on Math 1? Could you check where the Source is set to?

-Iris Xu

Or share a screenshot of the settings