pline --> start point --> enter A --> second point --> 3th point --> enter

Then the lisp will just work fine

 

If by "line" you mean an AutoCAD Line entity, that won't work.  It requires a Polyline made of two [and only two] arc segments [and only arc segments].  If you pick on anything else, whether any different entity type or a Polyline that doesn't match those requirements, OR if you simply missed in picking, or if you picked a qualifying Polyline but it's on a locked Layer, you'll get that message.  As suggested, post a drawing file [limited to just the Polyline in question] if you think everything qualifies.

I tried the lsp but just got the following

I converted the line to a LWPolyline but I still couldn't get it to work.  I am probably doing something wrong.  What am I doing wrong?  Thanks!

 

Give this a try!

Offset the Polyline to both sides by the desired distance [blue].

Type SNAPANG, and for the angle, Osnap ENDpoint to the mid-way vertices of any two Polylines, to get the skewed crosshairs [white].  One of the crosshair directions is the tangent direction of all 3 Polylines at their mid-way vertices.

ARC command, start at one of the mid-way vertices of a blue Polyline, use End option and pick a far end of the red Polyline, then use Direction option, and with ORTHO On, specify the tangent direction for the Arc [yellow] to head in from its start point.

Do the same 4 times, or if it's a symmetrical original Polyline, twice for one side and do some grip-edit Copying and Rotating for the other side.

If you want a single-Polyline result, use PEDIT's Join option or the JOIN command to connect the yellow Arcs.

 

The equivalent could be made into an AutoLisp routine, but first verify that this is the situation, and the result you need.

Hello MoeJoe, I don't know if the lisp is possible but when I draw with your method the last 2 polylines are with an angle in the middle.