So if you've accidentally drawn your lineart on the sketch layer in clip studio, there's this work around to fix it and get your lineart on it's own layer. This only works in clip studio as i dont think other programs have the 'convert brightness to opacity' tool

1) copy sketch layer to new layer
2) go to edit -> brightness/contrast and push both up to 100 to make the sketch white
3) go to edit -> 'convert brightness to opacity'
4) there you go! the lineart is now the only thing on the layer