So the Minecraft Development addon for Intelij adds many things however it handles the code part of things using Gradle/Maven (depending on which you selected), meaning that importing and stuff is handled by Intelij, not SpongeForge or anything like that.
This means that your Forge API code cannot be found if I cannot be imported (If you can import Forge without SpongeForge then you can with SpongeForge). Because you selected Forge when creating the project, what should have happened is in your maven “POM.xml” file or Gradle “build.gradle” file should have added the forge dependancy (as well as what forge requires such as MCP) to that file. So that is the first thing I would check (maybe a error occured when creating the project and that part got skipped for XYZ reason).
After that I would refresh the dependencies of gradle/maven (right click the project and then select maven then select “refresh dependencies” ->gradle run the “build --refresh-dependencies” command) which will redownload Forge and all required stuff.
If thats still not working, go into “project settings” and select your projects module, then select the dependies tab and check for Forge in there, if its not there then gradle/maven is failing which there are many reason for that. However if it is there, make sure the tag next to all dependencies is “provided” (except the ones that your plugin requires to be built with)
Hope that helps.