Theoretically anywhere, except in Abisko National Park - where you hike the last 15 km of Fjällräven Classic Sweden. There are regulations within the national park regarding where to camp or light a fire. These regulations must be observed. Within the national park you’re only allowed to camp or light a fire at a special tent location near Nissanjåkk, 5km from Abisko Tourist Station. You can also camp at Abiskojaure Cabin, in the National Park, but an extra tent-fee is applied.
You can find out more about the right of common access, which allows free camping under certain conditions, here.