There are four primary things to check in-game to ensure Sim Settlements 2 has loaded properly:
There is a magazine on the Sanctuary workbench that will instruct you to build, power, and activate a vanilla Recruitment Radio Beacon. The Sim Settlements 2 questline will start shortly after you do this.
Pro Tip: You can skip the magazine and just build, power, and activate a Recruitment Radio Beacon in any vanilla workshop you control to start the mod. It doesn't have to be in Sanctuary.
Alternatively, you can use the City Manager 2078 Holotape to skip the questline and unlock all of the Plot types, Building Plans, and other content. You'll find this under Tools > Cheats.
Fair warning: Skipping the questline may have unintended consequences with the functionality of your settlements.