Welcome to the forum and Archicad.
Firstly, please try to post in a relevant section of the forum and not just the 'Archicad+' section.
The section descriptions will hopefully help you choose the correct one.
By way of further explanation, if adjoining elements have the same surface material and their surfaces are in the same plane, then they will join seamlessly - no joining line - as they probably would appear in real life.
You will need to two different surface materials to see a join.
If you really need the same surface and still have a line, then you can duplicate the surface (in the surface settings dialogue) and give the second one a slightly different name.
Then use those surfaces.
One of the forum moderators. Versions 6.5 to 25 Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10 Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10