If LinearFit fails (which happens if your noise floor is too high), you need a manual PixelMath link. This is often required for overexposed stars or heavily vignetted edges.

If you prefer reading and following written steps:

Now that you have a high-resolution Luminance layer integrated, you can run to sharpen the stars and fine details. This is usually done before LRGB combination on the L channel alone, but can be done lightly on the final result.

The concept of a "Linear Link" is most commonly encountered in PixInsight’s Screen Transfer Function (STF)

Introduced in PixInsight 1.8.8, Local Normalization replaces the old “Linear Fit” clipping algorithm. LN corrects large-scale and small-scale background differences between sub-exposures caused by: