How can I insert lambda-bar symbol?

[Y.D.X]

Hi, welcome to the forum!

I don’t know this symbol, but I guess one of the following will satisfy you?

// U+019B Latin Extended-B LATIN SMALL LETTER LAMBDA WITH STROKE
$ ƛ $

// https://typst.app/docs/reference/math/underover/#functions-overline
$ overline(lambda) $

// https://typst.app/docs/reference/math/accent/
$ dash(lambda) $

// https://typst.app/docs/reference/text/strike/
#let lbar = {
  sym.wj
  strike(offset: -0.52em, extent: -0.05em, stroke: 0.5pt, "𝜆")
  sym.wj
}
$ lbar $