invert fillet

Hello
I'm new in librecad, is there a way to do invert fillet and How
Thanks

No.
The easiest workaround maybe to make a normal fillet first and then mirror it with it's endpoints as the mirror line points. If it's on a rectangle made with the rectangle command, explode it first, otherwise you can't select the fillet only.

i draw 2 lines (in yellow) and use that to make an inverted fillet