Short answer: No. In Sweden psilocybin is a controlled substance and attending or organizing a psilocybin retreat there is not legal. I cannot verify specific penalties, so check official sources if you need certainty.
If you want a legal option, consider traveling to a country where certain psilocybin activities are permitted. In the Netherlands some retreats operate using psilocybin truffles under current rules, but laws can change, so verify the latest regulations before booking. You can review upcoming group events at this page.
Do not arrange private sessions in Sweden. If considering a private session abroad, confirm that the provider, venue, and your travel comply with local law and safety requirements.
Stay safe and check the law before you book.