Hi,
>> I tried to do the spline on another drawing in the same
>> sheet works fine
Well, this means there does not exist a global issue.
What I see in your drawing ... your geometry is placed at very high coordinates. Before you do any more tests please move your geometry more close to 0,0,0

>> It should be in red and the box is in red too
I can find a red box, however there exist more than just 2 lines.
Please, first try the suggestion from above (move all more close to 0,0,0 and then test again) ... if this does not work, then please save the file with a new name, remove everything but these 2 lines you want to join and upload this drawing. This will make it more clear for me which 2 lines I have to test with.
>> I am using AutoCAD 2021
If command _ABOUT shows just "2021" then please install the latest service pack (which is currently 2021.1)
Also interesting: when I open your dwg-file I get the information that this dwg-file was created or modified by a non Autodesk product ... which product was involved to get this dwg-file created?
- alfred -
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Alfred NESWADBA
ISH-Solutions GmbH / Ingenieur Studio HOLLAUS
www.ish-solutions.at ...
blog.ish-solutions.at ...
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CDay 2026------------------------------------------------------------------------------------
(not an Autodesk consultant)