Generating a Multispectral image

Hello,

I’ve been able to generate a multispectral image in the past, but I’m now running into a problem where the software doesn’t let me select/highlight the standard reflectance of  my grey standard in the image. Usually, I just create a rectangle/ oval polygon/ where id like to designate my known reflectance value, but it doesn’t allow me to see my selection.  Is there anything I can do?

Thanks!

Nic