The builtin SetMouseSensitivity
console command unfortunately stops working with Enhanced Input System. You need to replace the old style mouse sensitivity configuration with an Input Modifier instead.
Adding custom mouse sensitivity logic
See Configuring mouse sensitivity in Enhanced Input
Adding a console command
Once you have sensitivity set up, you can create a custom console command f.ex. through the CheatManager class:
void UMyCheatManager::SetAimSensitivity(float Sensitivity)
{
auto* Subsystem = GetOuterAPlayerController()->GetLocalPlayer()->GetSubsystem<UEnhancedInputLocalPlayerSubsystem>();
Subsystem->GetUserSettings<UMyEnhancedInputUserSettings>()->SetAimSensitivity(Sensitivity, Sensitivity);
}