[linest] Whu LINEST can't the precise parameter of each variable?

Welcome to the board.

Because your data doesn't follow the formula.

Row\Col	A	B	C	D	E	F	G
2	x	y	z	d		x+y+z-1
3	1	0.6	0.4	0.8		-0.2	F3: =A3-B3+C3-1
4	1.2	1.1	0.8	0.9		-0.1
5	0.5	0.6	1	0.9		-0.1
6	0.7	0.9	0.8	0.6		-0.4

I think this is what you intended:

Row\Col	A	B	C	D	E
2	x	y	z	d
3	1	0.6	0.4	0.8
4	1.2	1.1	0.8	0.9
5	0.5	0.6	1	0.9
6	0.7	0.9	0.8	0.6
7
8	z	y	x
9	1	-1	1		A9:C9: {=LINEST(D3:D6, A3:C6,)}