LibreCad won't add a divided (half) ellipse to a polyline. Advice needed

I used the polyline tool to create a polygon shape. Then I tried to replace the bottom horizontal line segment with an upward-facing elliptical arc (looks like a hill rather than a valley) as follows:

1. Explode the polyline into individual segments
2. Delete the bottom segment
3. Create an ellipse with endpoints at the attachment points of the original horizontal line using the Ellipse (Axis) tool with Snap to Endpoints enabled
4. Divide the ellipse into top and bottom halves
5. Delete the bottom half of the ellipse
6. Use Tools>Polyline>Create Polyline from Existing Segments to reform the polyline

Everytime I try to add the half ellipse segment I get the response "No Entity found." I've tried trimming endpoints, zooming to huge scale to see if the end points aren't touching and I don't see any issues. All the other straight line segments add to the polyline without problems. It also won't let me create an offset on the half ellipse. Don't know if those two problems are connected. Anyone have a solution?

SOLVED by sand1024 by switching from ellipse to arc

well, so far the polyline may include just line and arc segments.

Elliptic arc could not be a part of polyline, this is limitation of DXF format (where polyline is described).

SOLVED
Thanks! I'm a beginner and wasn't aware of the arc tool. Fun fact: I was instructed through the ellipse method by an AI model. Human Help 1 AI 0