Why there's no left or rghit margin?

Hi, I try to adjust my margin to make as narrow as I can. But in layout option only top and bottom!!

Any idea ?

Dorico pro 3.5

In your screenshot, you’re showing two sets of margins: the music frame margins, which have only top and bottom margins (which are inside the page margins), and the page margins, which allow you to adjust the margins that the music frames sit within. See the Operation Manual here.