I did a test and the font (without installing it on the system) is visible in the preview window.
Windows 10, Sigil 1.8.0 + current master (two separate tests).
EDIT: I see problem in your styles – use normal (not qurly) quotes:
Code:
font-family: "Mekorot-Rashi";
font-weight: normal;
font-style: normal;
src: url("../Fonts/Mekorot-Rashi.ttf");