AI directs me to create an A6 card by opening a New Document > choose the Print option (I don’t readily see that)
Once again, AI probably gives you advices based on the previous V2 version where the presets for new documents were divided in Printer (for home/small business printers) which require RGB documents and Press-Ready (for professional large scale printing) which require CMYK files.
Thus, chose or define any appropriate size for your paper (A6 is 148 mm × 105 mm, it's not anymore in the predefined formats) and chose RGB/8 colour format. Check also the appropriate icon for Portrait or Paysage orientation.

For tracing shapes, once you have selected the appropriate tool (like rectangle round corners), you have to click on the document and then drag, from one corner of the desired shape to the opposite corner. If you want to have a square and not a rectangle, keep the Shift-key down.

Windows functions don't seem to work well in Affinity. Ctrl + C then Ctrl + V doesn’t seem to work
When you copy-paste an existing object, the same object is pasted on the exact same place, so you don't see there's a new object — unless you look at the Layers panel I already recommended you to leave open and check regularly.
So you only need to copy, paste and move the selected object (called a layer). It will move the newly pasted object and leave the original in place.
If you want to repeat that multiple times, select the original, press ctrl-j and move it (= duplicate and move). Repeat ctrl-j as much you want.
Enregistrement de l’écran 2026-07-13 à 07.38.33.mp4