Trying to make a fillet between two line, but a message like this is showing in the command line: Lines or polyline are non coplanar?
Issue:

You are trying to TRIM, EXTEND or FILLET two lines in AutoCAD and are receiving an error Lines are non coplanar and the lines are not modified.

Causes:
One or more of the line verticies is not at the same elevation as the other verticies.

Solution for non coplanar between two Lines or polylines to be fillet :-
1- If its non coplanar with polylines fillet :- 


Select polyline which it is non coplanar to be fillet  then click right select properties then go to Elevation put 0 value  ( see p
hoto for polylines non coplanar properties -1).



How fix Polylines to be fillet are non coplanar in autocad
Photo for polyline non coplanar properties -1

2- If its non coplanar with lines to be fillet :- 


Select lines which it is non coplanar to be fillet then click right to select properties then go to End Z put 0 value  ( see photo for lines non coplanar properties -2).
How to fix Polylines to be fillet are non coplanar
Photo for lines non coplanar  properties-2



For more details about fillet  non coplanar between two Lines polylines