This should be simple

%
G17 G20 G54

o101 sub
g01 y2.675 f10
g01 #endX
g01 y-.375 f10
g01 z#z
g01 x#startX
o101 endsub

o101.call [#startX=.5] [#z=-.2] [#endX=.675]
o101 call [#startX=.5] ]#z=--.4] [#endX=..675]
%

The error message is: "3 - Unknown word starting with s"
It seems like subroutines do not have the keyword sub after the subroutine number. Even I can't misread the subroutine examples that badly, can I?

Named parameters have a slightly different format, #endX should be #<endX>
Named Parameters

Progress, I think.

%
G17 G20 G54

o101 sub
g01 y2.675 f10
g01 #<endX>
g01 y-.375 f10
g01 z#<z>
g01 x#<startX>
o101 endsub

o101 call [#<startX>=.5] [#<z>=-.2] [#<endX>=.675]
o101 call [#<startX>=.5] ]#<z>=--.4] [#<endX>=..675]
%

gives: "1 - Named parameter #<startx> not defined "

All my named parameters are #<name> And linuxcnc.org/docs/html/gcode/overview.html#_named_parameters says that they "spring into existence when they are assigned a value for the first time. "

another option 1
;************
o101 sub
g01 y2.675 f10
g01 x#3
g01 y-.375 f10
g01 z#2
g01 x#1
o101 endsub

G17 G20 G54
o101 call [.5] [-.2] [.675]
M2
;************


another option 2
;************
o101 sub
g01 y2.675 f10
g01 x#3
g01 y-.375 f10
g01 z#2
g01 x#1
o101 endsub

G17 G20 G54
#<startX>=.5
#<z_>=-.4
#<endX>=.675
o101 call [#<startX>] [#<z_>] [#<endX>]
M2
;************



another option 3
;************
o101 sub
g01 y2.675 f10
g01 x#<_endX>
g01 y-.375 f10
g01 z#<_z_>
g01 x#<_startX>
o101 endsub

G17 G20 G54
#<_startX>=.5
#<_z_>=-.4
#<_endX>=.675
o101 call
M2
;************

I think your problem is also the fact that you are defining your named parameters in a subprogram. For that to work I believe your parameters need to be "global named parameters". To make them global simply add an underscore to the start of the parameter name #<_name>.