Combining different note values in the same space

I'm writing a piece for classical guitar and I'm having difficulty adding notes of another value (duration) directly below existing notes without having the existing notes affected. In the screenshot attached, the first lower 'd' should be a whole note which is sustained through the entire measure (this is the correct traditional writing style for classical guitar), but the program won't allow me to enter any different value than what's above. How do I get around this?