The guide lines have been changed in 15. Read the PDF ArchiCAD 15 Reference Guide starting on page 52 - Guide Line Improvements.
You click on the orange orb (called a guide line handle) to activate the line...
You can now drag guidelines to new locations and more...
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB