Kabam had already announced earlier that the October solo event will only be available to those who have completed the September solo event. However, you don’t have to complete all the difficulties. The September solo event was (and still is) available in different difficulties depending on your progression level. You have to complete at least one of them to get Summoned Symbiote and Anti-Venom.
Once done, you will receive it via in-game message. So open the game and check your Special solo objective. Read this for Week 4 Objective (to complete them quickly).