OK I see the problem.
Your interior elevation is set to have a limited horizontal range.
You can change it to unlimited and change it back afterwards.
Or use the TAB key on the limit line to toggle to the other line.
Or grab that centre node and move the limit line slightly then you should be able to select the room boundary line which is the one you want to adjust.
Then adjust the limit back.
I am not sure why the boundary line can not also be the limit line but I am sure there is a reason.
I'll just stick to the standard section lines.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11