Intellij Spring Initializr not available
Although the asker here used IntelliJ Ultimate here, I'd like to point out that the Spring Initializer is only available in the ultimate edition, not in the community edition. If you are using the community version, there is no way to enable the Spring plugin here.
If you only have IntelliJ IDEA CE then you can still create Spring Boot applications. First generate your project directory using https://start.spring.io and then import into IntelliJ IDEA and off you go.
Please verify "Spring Boot" plugin is enabled.
In File > Settings > Plugins, you will probably find the Plugin unselected:
(The above a screenshot of Intellij IDEA 2018.1 Ultimate)