Cannot Fillet and more: lines are non coplanar errors

You cannot Fillet, Trim, Extend, Chamfer or Hatch in a file, LT reports that the lines you select are "non coplanar".
LT is still AutoCAD, a 3D program that has a Z height elevation: those 2D commands require your lines to all be on the same Z height, ideally set to O (zero).

Create a new toolbar button on a toolbar (preferably a custom toolbar on your own), and add the following macro (ideally just cut and paste from here, watch out for word wrap):

^C^C_UCS;;_move _all;;0,0,1e99;;_move _all;;0,0,-1e99;;_chprop;_all;;_thickness;0;;^M;

Now click on the toolbar button you just created, and select the entities in question (or your entire drawing if needed). Make sure your UCS is set to World. This routine will not convert 3D objects in your file to 2D entities. ◦

Cannot Fillet and more: lines are non coplanar errors