@ Michel > In Rekordbox, when in "Full" view, there are auto loops located near the top and on the left of the beat grid. Turn on your "Quantize" button so it is in green, and you can click an auto-loop of "4" "8" "16" or "32" beats. You can also use the "In" and "Out" buttons to set your own loops, and click the "Reloop" button to exit current loops, or reloop what you have selected.
Here is a snapshot below for you.
Thanks,
Foster