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. The only places where you are allowed to camp or light a fire is in the area around the Abiskojaure Cabins and at a special tent location near Nissanjåkk, five kilometers south of Abisko Tourist Station. At the Abiskojaure Cabins the camping fee is 150 SEK for members of the Swedish Tourist Association, for non-members the fee is 250 SEK. Sauna is included in the fee.
You can find out more about the right of common access, which allows free camping under certain conditions, here.